bug#6284: Crash in w32_wnd_proc at frame deletion

[Lennart Borgman]

Now I finally got a backtrace of this crash. Thanks for the (harsh)
reminders about running under gdb.

Here is part of the backtrace

  Program received signal SIGSEGV, Segmentation fault.
  [Switching to Thread 1836.0xef8]
  0x011cc376 in w32_wnd_proc (hwnd=0x1001ac, msg=32, wParam=1049004,
      lParam=33554433) at w32fns.c:4351
  4351              if (f->output_data.w32->hourglass_p && !menubar_in_use
  (gdb) bt
  #0  0x011cc376 in w32_wnd_proc (hwnd=0x1001ac, msg=32, wParam=1049004,
      lParam=33554433) at w32fns.c:4351
  #1  0x7e418734 in USER32!GetDC () from C:\WINDOWS\system32\user32.dll
  #2  0x001001ac in ?? ()
  #3  0x00000020 in ?? ()
  #4  0x001001ac in ?? ()
  #5  0x02000001 in ?? ()
  #6  0x011c9291 in post_character_message (hwnd=0x11c9291, msg=1049004,
      wParam=32, lParam=1049004, dwmodifiers=33554433) at w32fns.c:3049
  #7  0x7e418816 in USER32!GetDC () from C:\WINDOWS\system32\user32.dll
  #8  0x011c9291 in post_character_message (hwnd=0x0, msg=18649745,
      wParam=1049004, lParam=32, dwmodifiers=1049004) at w32fns.c:3049
  #9  0x7e428ea0 in USER32!DefWindowProcW () from C:\WINDOWS\system32\user32.dll
  #10 0x00000000 in ?? ()

  Lisp Backtrace:
  "delete-frame" (0x82af44)

It looks like the important part is line 4351. I tried to look at f:

  (gdb) print f
  $1 = (struct frame *) 0x0
  (gdb) print f->output_data
  Cannot access memory at address 0x104

Does this mean that f might be gone here and that there should be a
test for this?