lavf: don't select an attached picture as default stream for seeking.

libavformat/utils.c

@@ -1307,7 +1307,8 @@ int av_find_default_stream_index(AVFormatContext *s)
         return -1;
     for(i = 0; i < s->nb_streams; i++) {
         st = s->streams[i];
-        if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
+        if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO &&
+            !(st->disposition & AV_DISPOSITION_ATTACHED_PIC)) {
             return i;
         }
         if (first_audio_index < 0 && st->codec->codec_type == AVMEDIA_TYPE_AUDIO)