avformat/mpegts: fix teletext PTS when selecting teletext streams only

avformat/mpegts: fix teletext PTS when selecting teletext streams only

After a1b4f120c031e6697bac9fd8c725d9c37ee36d13 the teletext PTS values were set
to AV_NOPTS_VALUE if the stream of the PCR pid was discarded.

What actually matters is that if we parse the PCR of the PCR PID or not, so
let's use the cached discard value of the actual PCR PID instead of the stream
discard value, which may be different.

Also fixes ticket #7567, which was caused by the fact that teletext PTS values
were not touched if the PCR pid was discarded even before
a1b4f120c031e6697bac9fd8c725d9c37ee36d13.

Signed-off-by: Marton Balint <cus@passwd.hu>