xwm: Set window->surface to NULL when surface goes away

Otherwise we may end up trying to remove the destroy listener if we
get the surface_destroy callback before the unmap_notify.

src/xwayland/window-manager.c

@@ -1847,6 +1847,8 @@ surface_destroy(struct wl_listener *listener, void *data)
 			     struct weston_wm_window, surface_destroy_listener);
 
 	wm_log("surface for xid %d destroyed\n", window->id);
+
+	window->surface = NULL;
 }
 
 static struct weston_wm_window *