- Fix skipsize on _update_backlog failure.
- Add robustness to AU completion detection by using AUD when present. If we've
received a AUD we overwrite the first VCL NAL detection when the result was
negative. VCL following AUD is the first VCL of next AU.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5862>