compositor: Only check buffer for NULL tried to allocate something

In particular, if we get a NULL buffer attach, it's ok for buffer to be NULL.

src/compositor.c

@@ -1442,12 +1442,13 @@ surface_attach(struct wl_client *client,
 	struct weston_surface *surface = wl_resource_get_user_data(resource);
 	struct weston_buffer *buffer = NULL;
 
-	if (buffer_resource)
+	if (buffer_resource) {
 		buffer = weston_buffer_from_resource(buffer_resource);
 		if (buffer == NULL) {
 			wl_client_post_no_memory(client);
 			return;
 		}
+	}
 
 	/* Attach, attach, without commit in between does not send
 	 * wl_buffer.release. */