xwayland: Better handle a race with X

If X windows are created and destroyed very fast sometimes the WM window
object gets created and destroyed before we get around to handling client
messages.  Failing to check that the window is still valid can result in a
segfault.

xwayland/window-manager.c

@@ -1403,6 +1403,12 @@ weston_wm_handle_client_message(struct weston_wm *wm,
 	       client_message->data.data32[4],
 	       client_message->window);
 
+	/* The window may get created and destroyed before we actually
+	 * handle the message.  If it doesn't exist, bail.
+	 */
+	if (!window)
+		return;
+
 	if (client_message->type == wm->atom.net_wm_moveresize)
 		weston_wm_window_handle_moveresize(window, client_message);
 	else if (client_message->type == wm->atom.net_wm_state)