Fix possible crash in server-decoration when surface destroyed

If the underlying surface is destroyed, but the client has not yet
destroyed the server decoration object, and then tries to call
request_mode() on it, the compositor will crash, because the
wlr_server_decoration struct has been freed, and the wl_resource's
user_data member has been NULLed out.

Yes, this is certainly an error for the client to do that, but I
shouldn't be able to write a buggy (or malicious) Wayland app that can
take down the entire compositor.

types/wlr_server_decoration.c

@@ -24,7 +24,7 @@ static void server_decoration_handle_request_mode(struct wl_client *client,
 		struct wl_resource *resource, uint32_t mode) {
 	struct wlr_server_decoration *decoration =
 		decoration_from_resource(resource);
-	if (decoration->mode == mode) {
+	if (decoration == NULL || decoration->mode == mode) {
 		return;
 	}
 	decoration->mode = mode;