mp4, 3gp field is reserved and value is 2

Originally committed as revision 5947 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/movenc.c

@@ -360,7 +360,7 @@ static int mov_write_audio_tag(ByteIOContext *pb, MOVTrack* track)
     put_be16(pb, 0); /* Revision level */
     put_be32(pb, 0); /* Reserved */
 
-    put_be16(pb, track->enc->channels); /* Number of channels */
+    put_be16(pb, track->mode == MODE_MOV ? track->enc->channels : 2); /* Number of channels */
     /* TODO: Currently hard-coded to 16-bit, there doesn't seem
                  to be a good way to get number of bits of audio */
     put_be16(pb, 0x10); /* Reserved */