mpegts: Share the cleanup code between the demuxer and lavf-internal parser functions

The lavf-internal parser functions are used when receiving
mpegts over RTP. This fixes memory leaks in this setup.

The normal mpegts demuxer close function was updated in ec7d0d2e in
2004 to fix leaks, but the parsing function used for RTP wasn't
updated and has been leaking ever since.

Signed-off-by: Martin Storsjö <martin@martin.st>
