gdb: fix regression in copy_type_recursive

gdb: fix regression in copy_type_recursive

Commit 5b7d941b90d1 ("gdb: add owner-related methods to struct type")
introduced a regression when running gdb.base/jit-reader-simple.exp and
others.  A NULL pointer dereference happens here:

    #3  0x0000557b7e9e8650 in gdbarch_obstack (arch=0x0) at /home/simark/src/binutils-gdb/gdb/gdbarch.c:484
    #4  0x0000557b7ea5b138 in copy_type_recursive (objfile=0x614000006640, type=0x62100018da80, copied_types=0x62100018e280) at /home/simark/src/binutils-gdb/gdb/gdbtypes.c:5537
    #5  0x0000557b7ea5dcbb in copy_type_recursive (objfile=0x614000006640, type=0x62100018e200, copied_types=0x62100018e280) at /home/simark/src/binutils-gdb/gdb/gdbtypes.c:5598
    #6  0x0000557b802cef51 in preserve_one_value (value=0x6110000b3640, objfile=0x614000006640, copied_types=0x62100018e280) at /home/simark/src/binutils-gdb/gdb/value.c:2518
    #7  0x0000557b802cf787 in preserve_values (objfile=0x614000006640) at /home/simark/src/binutils-gdb/gdb/value.c:2562
    #8  0x0000557b7fbaf19b in reread_symbols () at /home/simark/src/binutils-gdb/gdb/symfile.c:2489
    #9  0x0000557b7ec65d1d in run_command_1 (args=0x0, from_tty=1, run_how=RUN_NORMAL) at /home/simark/src/binutils-gdb/gdb/infcmd.c:439
    #10 0x0000557b7ec67a97 in run_command (args=0x0, from_tty=1) at /home/simark/src/binutils-gdb/gdb/infcmd.c:546

This is inside a TYPE_ALLOC macro.  The fact that gdbarch_obstack is
called means that the type is flagged as being arch-owned, but arch=0x0
means that type::arch returned NULL, probably meaning that the m_owner
field contains NULL.

If we look at the code before the problematic patch, in the
copy_type_recursive function, we see:

    if (! TYPE_OBJFILE_OWNED (type))
      return type;

    ...

    TYPE_OBJFILE_OWNED (new_type) = 0;
    TYPE_OWNER (new_type).gdbarch = get_type_arch (type);

The last two lines were replaced with:

    new_type->set_owner (type->arch ());

get_type_arch and type->arch isn't the same thing: get_type_arch gets
the type's arch owner if it is arch-owned, and gets the objfile's arch
if the type is objfile owned.  So it always returns non-NULL.
type->arch returns the type's arch if the type is arch-owned, else NULL.
So since the original type is objfile owned, it effectively made the new
type arch-owned (that is good) but set the owner to NULL (that is bad).

Fix this by using get_type_arch again there.

I spotted one other similar change in lookup_array_range_type, in the
original patch.  But that one appears to be correct, as it is executed
only if the type is arch-owned.

Add some asserts in type::set_owner to ensure we never set a NULL owner.
That would have helped catch the issue a little bit earlier, so it could
help in the future.

gdb/ChangeLog:

	* gdbtypes.c (copy_type_recursive): Use get_type_arch.
	* gdbtypes.h (struct type) <set_owner>: Add asserts.

Change-Id: I5d8bc7bfc83b3abc579be0b5aadeae4241179a00