avformat/mm: Check for existence of audio stream

No audio stream is created unconditionally and if none has been created,
no packet with stream_index 1 may be returned. This fixes an assert in
ff_read_packet() in libavformat/utils reported in ticket #8782.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit ec59dc73f0)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/mm.c

@@ -175,6 +175,8 @@ static int read_packet(AVFormatContext *s,
             return 0;
 
         case MM_TYPE_AUDIO :
+            if (s->nb_streams < 2)
+                return AVERROR_INVALIDDATA;
             if ((ret = av_get_packet(s->pb, pkt, length)) < 0)
                 return ret;
             pkt->stream_index = 1;