crash after killing some ediff buffers.

maybe_mark_black(void * 0x010eca23) line 1970 + 55 bytes
kkcc_marking(int 17755141) line 1054 + 6 bytes
gc_incremental() line 1975 + 421 bytes
Ffuncall(int 17363635, long * 0x00000001) line 3826
call1(long 37592232, long 37707920) line 4545 + 46 bytes
execute_internal_event(long 17408791) line 3103 + 10 bytes
Fdispatch_event(long 17173322) line 4644 + 6 bytes
Fcommand_loop_1() line 600 + 10 bytes
condition_case_1(long 37705216, long (long)* 0x010609c0 command_loop_1(long), long 37707920, long (long, long)* 0x010606e0 cmd_error(long, long), long 37707920) line 1924 + 44 bytes
command_loop_2(long 37707920) line 274 + 54 bytes
internal_catch(long 37456272, long (long)* 0x01060800 command_loop_2(long), long 37707920, int * volatile 0x00000000, long * volatile 0x00000000, long * volatile 0x00000000) line 1530 + 7 bytes
initial_command_loop(long 37707920) line 319
xemacs_21_5_b27_i586_pc_win32(int 0, unsigned short * * 0x00000000, unsigned short * * 0x00000000, int 0) line 1159
main(int 1, char * * 0x00f64130, char * * 0x00f62e10) line 3114
mainCRTStartup() line 338 + 17 bytes
KERNEL32! 7c816d4f()