rtpdec: Return AVERROR(EAGAIN) for mpegts parsing errors

This indicates that there was no error that needs to be reported to the
caller, so we can move on to parse the next packet immediately, if
available. The only error code that ff_mpegts_parse_packet can return
indicates that there was no packet to return from the provided data, for
which it returns -1.

Originally committed as revision 25496 to svn://svn.ffmpeg.org/ffmpeg/trunk