skip 2 bytes more when parsing ac3 frame, syncword 16bits + crc 16bits,

patch by Damiano Galassi, damiog at gmail dot com

Originally committed as revision 15196 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/movenc.c

@@ -233,7 +233,7 @@ static int mov_write_ac3_tag(ByteIOContext *pb, MOVTrack *track)
     put_be32(pb, 11);
     put_tag(pb, "dac3");
 
-    init_get_bits(&gbc, track->vosData+2, track->vosLen-2);
+    init_get_bits(&gbc, track->vosData+4, track->vosLen-4);
     fscod      = get_bits(&gbc, 2);
     frmsizecod = get_bits(&gbc, 6);
     bsid       = get_bits(&gbc, 5);