bug#14970: crash deleting frames

[Juanma Barranquero <lekktu@gmail.com>]

A crash, while closing a maximized frame.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 7104.0x1f30]
0x011e08b1 in w32_wnd_proc (hwnd=0x6c088c, msg=269, wParam=0,
lParam=0) at w32fns.c:3216
3216              w = XWINDOW (FRAME_SELECTED_WINDOW (f));
(gdb) bt
#0  0x011e08b1 in w32_wnd_proc (hwnd=0x6c088c, msg=269, wParam=0,
lParam=0) at w32fns.c:3216
#1  0x762162fa in USER32!OffsetRect () from C:\Windows\syswow64\user32.dll
#2  0x006c088c in ?? ()
#3  0x0000010d in ?? ()
#4  0x00000000 in ?? ()

Lisp Backtrace:
"delete-frame" (0x88efe8)
"handle-delete-frame" (0x88f324)
"call-interactively" (0x88f550)
"command-execute" (0x88f850)
(gdb) bt full
#0  0x011e08b1 in w32_wnd_proc (hwnd=0x6c088c, msg=269, wParam=0,
lParam=0) at w32fns.c:3216
        form = {
          dwStyle = 1,
          ptCurrentPos = {
            x = 8,
            y = 45
          },
          rcArea = {
            left = 8,
            top = 45,
            right = 1872,
            bottom = 540
          }
        }
        context = 0
        w = 0x6e4bfd68
        f = 0x0
        dpyinfo = 0x1523900
        wmsg = {
          msg = {
            hwnd = 0xf70828,
            message = 1981903226,
            wParam = 735304265,
            lParam = 1981903158,
            time = 16189480,
            pt = {
              x = 7080076,
              y = 1981903226
            }
          },
          dwModifiers = 16205752,
          rect = {
            left = 16205752,
            top = 0,
            right = 70,
            bottom = 1850473688
          }
        }
        windows_translate = 1850474000
        key = 1
#1  0x762162fa in USER32!OffsetRect () from C:\Windows\syswow64\user32.dll
No symbol table info available.
#2  0x006c088c in ?? ()
No symbol table info available.
#3  0x0000010d in ?? ()
No symbol table info available.
#4  0x00000000 in ?? ()
No symbol table info available.