avformat/avidec: Fix demuxing avi with broken index

Fixes: genio-avi
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avidec.c

@@ -1681,9 +1681,13 @@ static int guess_ni_flag(AVFormatContext *s)
 
         if (n >= 2) {
             int64_t pos = st->index_entries[0].pos;
-            avio_seek(s->pb, pos + 4, SEEK_SET);
+            unsigned tag[2];
+            avio_seek(s->pb, pos, SEEK_SET);
+            tag[0] = avio_r8(s->pb);
+            tag[1] = avio_r8(s->pb);
+            avio_rl16(s->pb);
             size = avio_rl32(s->pb);
-            if (pos + size > st->index_entries[1].pos)
+            if (get_stream_idx(tag) == i && pos + size > st->index_entries[1].pos)
                 last_start = INT64_MAX;
         }