winex11: Allow installing the event handler multiple times.

dlls/winex11.drv/event.c

@@ -186,7 +186,7 @@ static inline void free_event_data( XEvent *event )
 void X11DRV_register_event_handler( int type, x11drv_event_handler handler, const char *name )
 {
     assert( type <= MAX_EVENT_HANDLERS );
-    assert( !handlers[type] );
+    assert( !handlers[type] || handlers[type] == handler );
     handlers[type] = handler;
     event_names[type] = name;
     TRACE("registered handler %p for event %d %s\n", handler, type, debugstr_a(name) );