xdg-output: destroy outputs before manager

Since output_destroy() calls wl_list_remove() on the output's link,
the manager must still be valid. This is the same bug fixed in bf926e3
but with a different interface.

types/wlr_xdg_output_v1.c

@@ -227,6 +227,10 @@ static void handle_layout_change(struct wl_listener *listener, void *data) {
 }
 
 static void manager_destroy(struct wlr_xdg_output_manager_v1 *manager) {
+	struct wlr_xdg_output_v1 *output, *tmp;
+	wl_list_for_each_safe(output, tmp, &manager->outputs, link) {
+		output_destroy(output);
+	}
 	wlr_signal_emit_safe(&manager->events.destroy, manager);
 	wl_list_remove(&manager->display_destroy.link);
 	wl_list_remove(&manager->layout_add.link);