splitmuxsink: Catch invalid DTS to avoid running into problems later
DTS > PTS makes no sense, so we clamp DTS to the PTS. Also if there's a PTS but no DTS, then assume that PTS=DTS to make sure we're not working with a much older DTS.
DTS > PTS makes no sense, so we clamp DTS to the PTS. Also if there's a PTS but no DTS, then assume that PTS=DTS to make sure we're not working with a much older DTS.