matroska_read_seek: Fix used streams for subtitle index compensation

Might fix Ticket1907 (I have no testcase so i cant test)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -2508,10 +2508,11 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
         if (tracks[i].type == MATROSKA_TRACK_TYPE_SUBTITLE
             && tracks[i].stream->discard != AVDISCARD_ALL) {
             index_sub = av_index_search_timestamp(tracks[i].stream, st->index_entries[index].timestamp, AVSEEK_FLAG_BACKWARD);
-            if (index_sub >= 0
-                && st->index_entries[index_sub].pos < st->index_entries[index_min].pos
-                && st->index_entries[index].timestamp - st->index_entries[index_sub].timestamp < 30000000000/matroska->time_scale)
-                index_min = index_sub;
+            while(index_sub >= 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--;
         }
     }