matroskadec: matroska_read_seek after after EBML_STOP leads to failure.

EBML_STOP leaves matroska->current_id set. Then matroska_read_seek changes
the stream position without resetting current_id.  The next
matroska_parse_cluster  fails due to calculation of incorrect pos.  So clear
current_id when avio_seek happens in matroska_read_seek.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/matroskadec.c

@@ -1904,6 +1904,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
 
     if ((index = av_index_search_timestamp(st, timestamp, flags)) < 0) {
         avio_seek(s->pb, st->index_entries[st->nb_index_entries-1].pos, SEEK_SET);
+        matroska->current_id = 0;
         while ((index = av_index_search_timestamp(st, timestamp, flags)) < 0) {
             matroska_clear_queue(matroska);
             if (matroska_parse_cluster(matroska) < 0)
@@ -1932,6 +1933,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
     }
 
     avio_seek(s->pb, st->index_entries[index_min].pos, SEEK_SET);
+    matroska->current_id = 0;
     matroska->skip_to_keyframe = !(flags & AVSEEK_FLAG_ANY);
     matroska->skip_to_timecode = st->index_entries[index].timestamp;
     matroska->done = 0;