dix: set dev->key to NULL after freeing it in CloseDevice. (Bug #12830)

Otherwise XkbRemoveResourceClient may try to dereference it lateron.

X.Org Bug 12830 <http://bugs.freedesktop.org/show_bug.cgi?id=12830>

dix/devices.c

@@ -529,6 +529,7 @@ CloseDevice(DeviceIntPtr dev)
 	xfree(dev->key->curKeySyms.map);
 	xfree(dev->key->modifierKeyMap);
 	xfree(dev->key);
+        dev->key = NULL;
     }
 
     if (dev->valuator) {