mieq: Fix a crash regression in mieqProcessDeviceEvent

(lldb) bt
* thread #6: tid = 0x92d4eb, 0x00000001001dee94 X11.bin`mieqProcessDeviceEvent(dev=0x0000000000000000, event=0x0000000100298bb0,
screen=0x0000000000000000) + 36 at mieq.c:519, stop reason = EXC_BAD_ACCESS (code=1, address=0x44)
  * frame #0: 0x00000001001dee94 X11.bin`mieqProcessDeviceEvent(dev=0x0000000000000000, event=0x0000000100298bb0, screen=0x0000000000000000) + 36 at
mieq.c:519
    frame #1: 0x00000001001df3eb X11.bin`mieqProcessInputEvents + 555 at mieq.c:631
    frame #2: 0x0000000100017674 X11.bin`ProcessInputEvents + 20 at darwinEvents.c:422
    frame #3: 0x0000000100175eaa X11.bin`Dispatch + 154 at dispatch.c:357
    frame #4: 0x0000000100181b4a X11.bin`dix_main(argc=4, argv=0x00007fff5fbff750, envp=0x00007fff5fbff650) + 1594 at main.c:296
    frame #5: 0x000000010001ba80 X11.bin`server_thread(arg=0x0000000101208220) + 64 at quartzStartup.c:66
    frame #6: 0x00007fff89bb9899 libsystem_pthread.dylib`_pthread_body + 138
    frame #7: 0x00007fff89bb972a libsystem_pthread.dylib`_pthread_start + 137
    frame #8: 0x00007fff89bbdfc9 libsystem_pthread.dylib`thread_start + 13

Regression from: 9fb08310b51b46736f3ca8dbc04efdf502420403

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 1faa76670572e3478965fd2cd9ab60ab2d865e3a)