avformat/mpegts: unset DTS/PTS for subtitle PES packets if PCR not available

Fixes issues when a subtitle packet is received before PCR for the
program has been received, leading to wildly jumping timestamps
on the lavf client side as well as in the re-ordering logic.

This usually happens in case of multiplexes where the PCR of a
program is not taken into account with subtitle tracks' DTS/PTS.

libavformat/mpegts.c

@@ -1219,6 +1219,7 @@ skip:
                         || pes->st->codecpar->codec_id == AV_CODEC_ID_DVB_SUBTITLE)
                     ) {
                     AVProgram *p = NULL;
+                    int pcr_found = 0;
                     while ((p = av_find_program_from_stream(pes->stream, p, pes->st->index))) {
                         if (p->pcr_pid != -1 && p->discard != AVDISCARD_ALL) {
                             MpegTSFilter *f = pes->ts->pids[p->pcr_pid];
@@ -1242,6 +1243,7 @@ skip:
                                     // 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 (pes->dts == AV_NOPTS_VALUE || pes->dts < pcr) {
@@ -1258,6 +1260,15 @@ skip:
                             }
                         }
                     }
+
+                    if (!pcr_found) {
+                        av_log(pes->stream, AV_LOG_VERBOSE,
+                               "Forcing DTS/PTS to be unset for a "
+                               "non-trustworthy PES packet for PID %d as "
+                               "PCR hasn't been received yet.\n",
+                               pes->pid);
+                        pes->dts = pes->pts = AV_NOPTS_VALUE;
+                    }
                 }
             }
             break;