[gdb] Fix assertion failure due to null frame

[gdb] Fix assertion failure due to null frame

PR gdb/33512 reports an assertion failure in test-case
gdb.ada/access_to_packed_array.exp on i386-linux:
...
(gdb) maint print symbols
gdb/frame.c:3400: internal-error: reinflate: \
  Assertion `m_cached_level >= -1' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) FAIL: $exp: \
  maint print symbols (GDB internal error)
...

I haven't been able to reproduce the failure by running the test-case on
x86_64-linux with target board unix/-m32, but I'm able to reproduce on
x86_64-linux by using the exec attached to the PR:
...
$ cat gdb.in
file foo
maint expand-symtabs
maint print symbols
$ gdb -q -batch -ex "set trace-commands on" -x gdb.in
   ...
         c_to: array (gdb/frame.c:3395: internal-error: reinflate: \
	                Assertion `m_cached_level >= -1' failed.
...

The problem happens when trying to print variable c_to:
...
 <4><f227>: Abbrev Number: 3 (DW_TAG_variable)
    <f228>   DW_AT_name        : c_to
    <f230>   DW_AT_type        : <0xf214>
...
with type:
...
 <4><f214>: Abbrev Number: 7 (DW_TAG_array_type)
    <f215>   DW_AT_type        : <0x9f39>
 <5><f21d>: Abbrev Number: 12 (DW_TAG_subrange_type)
    <f21e>   DW_AT_type        : <0x9d6c>
    <f222>   DW_AT_upper_bound : <0xf209>
...
with upper bound:
...
 <4><f209>: Abbrev Number: 89 (DW_TAG_variable)
    <f20a>   DW_AT_name        : system__os_lib__copy_file__copy_to__TTc_toSP1___U
    <f20e>   DW_AT_type        : <0x9d6c>
    <f212>   DW_AT_artificial  : 1
    <f212>   DW_AT_location    : 1 byte block: 57       (DW_OP_reg7 (edi))
...

The backtrace at the point of the assertion failure is:
...
 (gdb) bt
 #0  __pthread_kill_implementation (threadid=<optimized out>,
     signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
 #1  0x00007ffff62a8e7f in __pthread_kill_internal (signo=6,
     threadid=<optimized out>) at pthread_kill.c:78
 #2  0x00007ffff6257842 in __GI_raise (sig=sig@entry=6)
     at ../sysdeps/posix/raise.c:26
 #3  0x00007ffff623f5cf in __GI_abort () at abort.c:79
 #4  0x00000000010e7ac6 in dump_core () at gdb/utils.c:223
 #5  0x00000000010e81b8 in internal_vproblem(internal_problem *, const char *, int, const char *, typedef __va_list_tag __va_list_tag *) (
     problem=0x2ceb0c0 <internal_error_problem>,
     file=0x1ad5a90 "gdb/frame.c", line=3395,
     fmt=0x1ad5a08 "%s: Assertion `%s' failed.", ap=0x7fffffffc3c0)
     at gdb/utils.c:475
 #6  0x00000000010e82ac in internal_verror (
     file=0x1ad5a90 "gdb/frame.c", line=3395,
     fmt=0x1ad5a08 "%s: Assertion `%s' failed.", ap=0x7fffffffc3c0)
     at gdb/utils.c:501
 #7  0x00000000019be79f in internal_error_loc (
     file=0x1ad5a90 "gdb/frame.c", line=3395,
     fmt=0x1ad5a08 "%s: Assertion `%s' failed.")
     at gdbsupport/errors.cc:57
 #8  0x00000000009b5c16 in frame_info_ptr::reinflate (this=0x7fffffffc878)
     at gdb/frame.c:3395
 #9  0x00000000009b66f9 in frame_info_ptr::operator-> (this=0x7fffffffc878)
     at gdb/frame.h:290
 #10 0x00000000009b4bd5 in get_frame_arch (this_frame=...)
     at gdb/frame.c:3075
 #11 0x000000000081dd89 in dwarf_expr_context::fetch_result (
     this=0x7fffffffc810, type=0x410d600, subobj_type=0x410d600,
     subobj_offset=0, as_lval=true)
     at gdb/dwarf2/expr.c:1006
 #12 0x000000000081e2ef in dwarf_expr_context::evaluate (this=0x7fffffffc810,
     addr=0x7ffff459ce6b "W\aF\003", len=1, as_lval=true,
     per_cu=0x7fffd00053f0, frame=..., addr_info=0x7fffffffcc30, type=0x0,
     subobj_type=0x0, subobj_offset=0)
     at gdb/dwarf2/expr.c:1136
 #13 0x0000000000877c14 in dwarf2_locexpr_baton_eval (dlbaton=0x3e99c18,
     frame=..., addr_stack=0x7fffffffcc30, valp=0x7fffffffcab0,
     push_values=..., is_reference=0x7fffffffc9b0)
     at gdb/dwarf2/loc.c:1604
 #14 0x0000000000877f71 in dwarf2_evaluate_property (prop=0x3e99ce0,
     initial_frame=..., addr_stack=0x7fffffffcc30, value=0x7fffffffcab0,
     push_values=...) at gdb/dwarf2/loc.c:1668
 #15 0x00000000009def76 in resolve_dynamic_range (dyn_range_type=0x3e99c50,
     addr_stack=0x7fffffffcc30, frame=..., rank=0, resolve_p=true)
     at gdb/gdbtypes.c:2198
 #16 0x00000000009e0ded in resolve_dynamic_type_internal (type=0x3e99c50,
     addr_stack=0x7fffffffcc30, frame=..., top_level=true)
     at gdb/gdbtypes.c:2934
 #17 0x00000000009e1079 in resolve_dynamic_type (type=0x3e99c50, valaddr=...,
     addr=0, in_frame=0x0) at gdb/gdbtypes.c:2989
 #18 0x0000000000488ebc in ada_discrete_type_low_bound (type=0x3e99c50)
     at gdb/ada-lang.c:710
 #19 0x00000000004eb734 in print_range (type=0x3e99c50, stream=0x30157b0,
     bounds_preferred_p=0) at gdb/ada-typeprint.c:156
 #20 0x00000000004ebffe in print_array_type (type=0x3e99d10, stream=0x30157b0,
     show=1, level=9, flags=0x1bdcf20 <type_print_raw_options>)
     at gdb/ada-typeprint.c:381
 #21 0x00000000004eda3c in ada_print_type (type0=0x3e99d10,
     varstring=0x401f710 "c_to", stream=0x30157b0, show=1, level=9,
     flags=0x1bdcf20 <type_print_raw_options>)
     at gdb/ada-typeprint.c:1015
 #22 0x00000000004b4627 in ada_language::print_type (
     this=0x2f949b0 <ada_language_defn>, type=0x3e99d10,
     varstring=0x401f710 "c_to", stream=0x30157b0, show=1, level=9,
     flags=0x1bdcf20 <type_print_raw_options>)
     at gdb/ada-lang.c:13681
 #23 0x0000000000f74646 in print_symbol (gdbarch=0x3256270, symbol=0x3e99db0,
     depth=9, outfile=0x30157b0) at gdb/symmisc.c:545
 #24 0x0000000000f737e6 in dump_symtab_1 (symtab=0x3ddd7e0, outfile=0x30157b0)
     at gdb/symmisc.c:313
 #25 0x0000000000f73a69 in dump_symtab (symtab=0x3ddd7e0, outfile=0x30157b0)
     at gdb/symmisc.c:370
 #26 0x0000000000f7420f in maintenance_print_symbols (args=0x0, from_tty=0)
     at gdb/symmisc.c:481
 #27 0x00000000006c7fde in do_simple_func (args=0x0, from_tty=0, c=0x321e270)
     at gdb/cli/cli-decode.c:94
 #28 0x00000000006ce65a in cmd_func (cmd=0x321e270, args=0x0, from_tty=0)
     at gdb/cli/cli-decode.c:2826
 #29 0x0000000001005b78 in execute_command (p=0x3f48fe3 "", from_tty=0)
     at gdb/top.c:564
 #30 0x0000000000966095 in command_handler (
     command=0x3f48fd0 "maint print symbols")
     at gdb/event-top.c:613
 #31 0x0000000001005141 in read_command_file (stream=0x3011a40)
     at gdb/top.c:333
 #32 0x00000000006e2a64 in script_from_file (stream=0x3011a40,
     file=0x7fffffffe21f "gdb.in")
     at gdb/cli/cli-script.c:1705
 #33 0x00000000006bb88c in source_script_from_stream (stream=0x3011a40,
     file=0x7fffffffe21f "gdb.in", file_to_open=0x7fffffffd760 "gdb.in")
     at gdb/cli/cli-cmds.c:706
 #34 0x00000000006bba12 in source_script_with_search (
     file=0x7fffffffe21f "gdb.in", from_tty=0, search_path=0)
     at gdb/cli/cli-cmds.c:751
 #35 0x00000000006bbab2 in source_script (file=0x7fffffffe21f "gdb.in",
     from_tty=0) at gdb/cli/cli-cmds.c:760
 #36 0x0000000000b835cb in catch_command_errors (
     command=0x6bba7e <source_script(char const*, int)>,
     arg=0x7fffffffe21f "gdb.in", from_tty=0, do_bp_actions=false)
     at gdb/main.c:510
 #37 0x0000000000b83803 in execute_cmdargs (cmdarg_vec=0x7fffffffd980,
     file_type=CMDARG_FILE, cmd_type=CMDARG_COMMAND, ret=0x7fffffffd8c8)
     at gdb/main.c:606
 #38 0x0000000000b84d79 in captured_main_1 (context=0x7fffffffdb90)
     at gdb/main.c:1349
 #39 0x0000000000b84fe4 in captured_main (context=0x7fffffffdb90)
     at gdb/main.c:1372
 #40 0x0000000000b85092 in gdb_main (args=0x7fffffffdb90)
     at gdb/main.c:1401
 #41 0x000000000041a382 in main (argc=9, argv=0x7fffffffdcc8)
     at gdb/gdb.c:38
 (gdb)
...

The immediate problem is in dwarf_expr_context::fetch_result where we're
calling get_frame_arch:
...
      switch (this->m_location)
	{
	case DWARF_VALUE_REGISTER:
	  {
	    gdbarch *f_arch = get_frame_arch (this->m_frame);
...
with a null frame:
...
(gdb) p this->m_frame.is_null ()
$1 = true
(gdb)
...

Fix this using ensure_have_frame in dwarf_expr_context::execute_stack_op for
DW_OP_reg<n> and DW_OP_regx, getting us instead:
...
         c_to: array (<>) of character; computed at runtime
...

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33512