[gdb] Handle bad alloc handling in gdb_bfd_open

[gdb] Handle bad alloc handling in gdb_bfd_open

Say we simulate a bad alloc in gdb_bfd_init_data:
...
+  {
+    static bool throw_bad_alloc = true;
+    if (throw_bad_alloc)
+      {
+       throw_bad_alloc = false;
+
+       va_list dummy;
+       throw gdb_quit_bad_alloc (gdb_exception_quit ("bad alloc", dummy));
+      }
+  }
   gdata = new gdb_bfd_data (abfd, st);
...

That works out fine for doing "file a.out" once:
...
$ gdb -q -batch -ex "file a.out"
bad alloc
$
...
but doing so twice get us:
...
$ gdb -q -batch -ex "file a.out" -ex "file a.out"
bad alloc

Fatal signal: Segmentation fault
----- Backtrace -----
0x5183f7 gdb_internal_backtrace_1
        /home/vries/gdb/src/gdb/bt-utils.c:121
0x5183f7 _Z22gdb_internal_backtracev
        /home/vries/gdb/src/gdb/bt-utils.c:167
0x62329b handle_fatal_signal
        /home/vries/gdb/src/gdb/event-top.c:917
0x6233ef handle_sigsegv
        /home/vries/gdb/src/gdb/event-top.c:990
0xfffeffba483f ???
0x65554c eq_bfd
        /home/vries/gdb/src/gdb/gdb_bfd.c:231
0xeaca77 htab_find_with_hash
        /home/vries/gdb/src/libiberty/hashtab.c:597
0x657487 _Z12gdb_bfd_openPKcS0_ib
        /home/vries/gdb/src/gdb/gdb_bfd.c:580
0x6272d7 _Z16exec_file_attachPKci
        /home/vries/gdb/src/gdb/exec.c:451
0x627e67 exec_file_command
        /home/vries/gdb/src/gdb/exec.c:550
0x627f23 file_command
        /home/vries/gdb/src/gdb/exec.c:565
Segmentation fault (core dumped)
$
...

The problem is in gdb_bfd_open, where we insert abfd into gdb_bfd_cache:
...
  if (bfd_sharing)
    {
      slot = htab_find_slot_with_hash (gdb_bfd_cache, &search, hash, INSERT);
      gdb_assert (!*slot);
      *slot = abfd;
    }

  gdb_bfd_init_data (abfd, &st);
...
while the bad alloc means that gdb_bfd_init_data is interrupted and abfd is
not properly initialized.

Fix this by reversing the order, inserting abfd into gdb_bfd_cache only after
a successful call to gdb_bfd_init_data, such that we get:
...
$ gdb -q -batch -ex "file a.out" -ex "file a.out"
bad alloc
$
...

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>