avformat/matroskaenc: Remove redundant assert

The Matroska muxer groups index entries with the same pts together in
order to save a few bytes. Because of Matroska's variable-length length
fields, mkv_write_cues() does this by first finding out how many index
entries will be grouped together before actually writing them.
Currently, it is asserted at both of these stages that the stream index
of the list of designated index entries is valid. But the second assert
is redundant, because the very same index entries have already been
checked.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskaenc.c

@@ -592,7 +592,7 @@ static int64_t mkv_write_cues(AVFormatContext *s, mkv_cues *cues, mkv_track *tra
             tracks[j].has_cue = 0;
         for (j = 0; j < cues->num_entries - i && entry[j].pts == pts; j++) {
             int tracknum = entry[j].stream_idx;
-            av_assert0(tracknum>=0 && tracknum<num_tracks);
+
             if (tracks[tracknum].has_cue && s->streams[tracknum]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
                 continue;
             tracks[tracknum].has_cue = 1;