Fix wild pointer to buffer

After the buffer is released from the release event, it should no longer be
accessed.

Fixes a GTK crash.

types/wlr_surface.c

@@ -293,6 +293,7 @@ release:
 	pixman_region32_init(&surface->current.buffer_damage);
 
 	wl_resource_queue_event(surface->current.buffer, WL_BUFFER_RELEASE);
+	surface->current.buffer = NULL;
 }
 
 static void surface_set_buffer_transform(struct wl_client *client,