avformat/matroskadec: fix off by 1 error in matroska_read_seek()

Fixes out of array read

Fixes: vp9-opus-crash.webm

Found-by: Dale Curtis <dalecurtis@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -3018,7 +3018,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
                 tracks[i].stream, st->index_entries[index].timestamp,
                 AVSEEK_FLAG_BACKWARD);
             while (index_sub >= 0 &&
-                  index_min >= 0 &&
+                  index_min > 0 &&
                   tracks[i].stream->index_entries[index_sub].pos < st->index_entries[index_min].pos &&
                   st->index_entries[index].timestamp - tracks[i].stream->index_entries[index_sub].timestamp < 30000000000 / matroska->time_scale)
                 index_min--;