xwayland/window-manager: Avoid doing work for unknown window

In particular if the hash table lookup fails and returns NULL then that value
would be passed into weston_wm_window_schedule_repaint which does not accept a
NULL value.

src/xwayland/window-manager.c

@@ -800,7 +800,9 @@ weston_wm_handle_property_notify(struct weston_wm *wm, xcb_generic_event_t *even
 	struct weston_wm_window *window;
 
 	window = hash_table_lookup(wm->window_hash, property_notify->window);
-	if (window)
+	if (!window)
+		return;
+
 	window->properties_dirty = 1;
 
 	weston_log("XCB_PROPERTY_NOTIFY: window %d, ",