Avoid "unused variable 'c'" warning.

Originally committed as revision 1282 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mace.c

@@ -382,7 +382,6 @@ static void Exp1to6(MACEContext *ctx,
 
 static int mace_decode_init(AVCodecContext * avctx)
 {
-    MACEContext *c = avctx->priv_data;
     if (avctx->channels > 2)
         return -1;
     return 0;