blender/windowmanager/intern/wm_files.c:wm_window_match_do()
Fix crash on null pointer dereference of *screen

Steps to reproduce:
1. Start blender with factory settings.
2. Move the cube so the scene changes.
3. Quit
4. blender /tmp/quit.blend
5. Crash

Backtrace
Program received signal SIGSEGV, Segmentation fault.
wm_window_match_do (C=0x1eb6498, oldwmlist=0x7fffffffd490) at /usr/src/debug/blender-2.60/source/blender/windowmanager/intern/wm_files.c:207
207 if(screen->winid==0)
(gdb) bt
#0 wm_window_match_do (C=0x1eb6498, oldwmlist=0x7fffffffd490) at /usr/src/debug/blender-2.60/source/blender/windowmanager/intern/wm_files.c:207
#1 0x00000000006d28cc in WM_read_file (C=0x1eb6498, filepath=<optimized out>, reports=0x7fffffffd4f0)
at /usr/src/debug/blender-2.60/source/blender/windowmanager/intern/wm_files.c:387
#2 0x00000000006cb265 in load_file (UNUSED_argc=<optimized out>, argv=<optimized out>, data=0x1eb6498) at /usr/src/debug/blender-2.60/source/creator/creator.c:989
#3 0x0000000000b706a4 in BLI_argsParse (ba=0x2137908, pass=4, default_cb=0x6cb180 <load_file>, default_data=0x1eb6498)
at /usr/src/debug/blender-2.60/source/blender/blenlib/intern/BLI_args.c:283
#4 0x00000000006c50ee in main (argc=2, argv=0x7fffffffd7f8) at /usr/src/debug/blender-2.60/source/creator/creator.c:1246
(gdb)