movenc illegal access patch by Roine Gustafsson <roine AT users DOT sourceforge DOT net>

Originally committed as revision 3891 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/movenc.c

@@ -1096,7 +1096,7 @@ static int mov_write_udta_tag(ByteIOContext *pb, MOVContext* mov,
     }
 
     /* Encoder */
-    if(!(mov->tracks[0].enc->flags & CODEC_FLAG_BITEXACT))
+    if(mov->tracks[0].enc && !(mov->tracks[0].enc->flags & CODEC_FLAG_BITEXACT))
     {
         int pos = url_ftell(pb);
         put_be32(pb, 0); /* size */