shell: Fix set_transient_for with a NULL parent

desktop-shell/shell.c

@@ -1855,8 +1855,12 @@ xdg_surface_set_transient_for(struct wl_client *client,
                              struct wl_resource *parent_resource)
 {
 	struct shell_surface *shsurf = wl_resource_get_user_data(resource);
-	struct weston_surface *parent =
+	struct weston_surface *parent;
-		wl_resource_get_user_data(parent_resource);
+
+	if (parent_resource)
+		parent = wl_resource_get_user_data(parent_resource);
+	else
+		parent = NULL;
 
 	shell_surface_set_parent(shsurf, parent);
 }