avidec: try to support oddly muxed MMES stream

Fixes ticket1804
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avidec.c

@@ -1271,6 +1271,11 @@ static int avi_read_idx1(AVFormatContext *s, int size)
     avi->stream_index = -1;
     avio_seek(pb, idx1_pos, SEEK_SET);
 
+    if (s->nb_streams == 1 && s->streams[0]->codec->codec_tag == AV_RL32("MMES")){
+        first_packet_pos = 0;
+        data_offset = avi->movi_list;
+    }
+
     /* Read the entries and sort them in each stream component. */
     for(i = 0; i < nb_index_entries; i++) {
         if(url_feof(pb))