wayland/buffer: Do not get latest sync_fd if there are no release points

It's pointless busy work in that case.

Fixes: e8b890ab53 ("wayland: Implement linux-drm-syncobj-v1")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4208>

src/wayland/meta-wayland-buffer.c

@@ -731,6 +731,9 @@ meta_wayland_buffer_dec_use_count (MetaWaylandBuffer *buffer)
       if (buffer->resource)
         wl_buffer_send_release (buffer->resource);
 
+      if (!buffer->release_points->len)
+        return;
+
       sync_fd = cogl_context_get_latest_sync_fd (cogl_context);
       if (sync_fd < 0)
         {