avformat/mpegts: only reset timestamps to NOPTS for DVB teletext

While having the possibility of non-NOPTS values that can suddenly
jump in time due to adjustments to match PCR is not nice for DVB
subtitles, apparently the parser for this format bases its behavior on
whether the packets' timestamps are NOPTS or not. Thus while we can
adjust timestamps, we should exclude DVB subtitles from the timestamp
unsetting logic.

Fixes #8844

libavformat/mpegts.c

@@ -1343,7 +1343,8 @@ skip:
                         }
                     }
 
-                    if (!pcr_found) {
+                    if (pes->st->codecpar->codec_id == AV_CODEC_ID_DVB_TELETEXT &&
+                        !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 "