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

After a1b4f120c0 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
a1b4f120c0.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit 765c56bfa9)

libavformat/mpegts.c

@@ -1303,15 +1303,17 @@ skip:
                                             st = pst;
                                     }
                                 }
-                                if (f->last_pcr != -1 && st && st->discard != AVDISCARD_ALL) {
+                                if (f->last_pcr != -1 && !f->discard) {
                                     // teletext packets do not always have correct timestamps,
                                     // the standard says they should be handled after 40.6 ms at most,
                                     // and the pcr error to this packet should be no more than 100 ms.
                                     // TODO: we should interpolate the PCR, not just use the last one
                                     int64_t pcr = f->last_pcr / 300;
                                     pcr_found = 1;
-                                    pes->st->pts_wrap_reference = st->pts_wrap_reference;
-                                    pes->st->pts_wrap_behavior = st->pts_wrap_behavior;
+                                    if (st) {
+                                        pes->st->pts_wrap_reference = st->pts_wrap_reference;
+                                        pes->st->pts_wrap_behavior = st->pts_wrap_behavior;
+                                    }
                                     if (pes->dts == AV_NOPTS_VALUE || pes->dts < pcr) {
                                         pes->pts = pes->dts = pcr;
                                     } else if (pes->st->codecpar->codec_id == AV_CODEC_ID_DVB_TELETEXT &&