mpegts: When playing mpegts over rtp/rtsp/sdp, lavf must still read PAT/SDT to get available streams

The code path using for mpegts over rtp doesn't open the demuxer using
mpegts_read_header,
so it never starts listening for PAT/SDT, only uses auto_guess
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>