backend/wayland: fix segfault in wlr_wl_output_destroy

backend/wayland/output.c

@@ -18,10 +18,12 @@ int os_create_anonymous_file(off_t size);
 static struct wl_callback_listener frame_listener;
 
 static void surface_frame_callback(void *data, struct wl_callback *cb, uint32_t time) {
-	struct wlr_output *wlr_output = data;
+	struct wlr_wl_backend_output *output = data;
+	struct wlr_output *wlr_output = (struct wlr_output *)output;
 	assert(wlr_output);
 	wl_signal_emit(&wlr_output->events.frame, wlr_output);
 	wl_callback_destroy(cb);
+	output->frame_callback = NULL;
 }
 
 static struct wl_callback_listener frame_listener = {