mpegts: check substreams before discarding

Allow to extract the AC3 core from TrueHD with the "copy" codec.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/mpegts.c

@@ -784,7 +784,8 @@ static int mpegts_push_data(MpegTSFilter *filter,
                     code = pes->header[3] | 0x100;
                     av_dlog(pes->stream, "pid=%x pes_code=%#x\n", pes->pid, code);
 
-                    if ((pes->st && pes->st->discard == AVDISCARD_ALL) ||
+                    if ((pes->st && pes->st->discard == AVDISCARD_ALL &&
+                         (!pes->sub_st || pes->sub_st->discard == AVDISCARD_ALL)) ||
                         code == 0x1be) /* padding_stream */
                         goto skip;