server: When merging mouse messages ignore the window if it isn't set.

server/queue.c

@@ -407,7 +407,7 @@ static int merge_message( struct thread_input *input, const struct message *msg
     if (!ptr) return 0;
     prev = LIST_ENTRY( ptr, struct message, entry );
     if (prev->result) return 0;
-    if (prev->win != msg->win) return 0;
+    if (prev->win && msg->win && prev->win != msg->win) return 0;
     if (prev->msg != msg->msg) return 0;
     if (prev->type != msg->type) return 0;
     /* now we can merge it */