ffv1dec: use correct linesize

Apparently this did not break anything.
Signed-off-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ffv1.c

@@ -1435,7 +1435,7 @@ static int decode_slice(AVCodecContext *c, void *arg){
         decode_plane(fs, p->data[0] + ps*x + y*p->linesize[0], width, height, p->linesize[0], 0);
 
         decode_plane(fs, p->data[1] + ps*cx+cy*p->linesize[1], chroma_width, chroma_height, p->linesize[1], 1);
-        decode_plane(fs, p->data[2] + ps*cx+cy*p->linesize[1], chroma_width, chroma_height, p->linesize[2], 1);
+        decode_plane(fs, p->data[2] + ps*cx+cy*p->linesize[2], chroma_width, chroma_height, p->linesize[2], 1);
         if (fs->transparency)
             decode_plane(fs, p->data[3] + ps*x + y*p->linesize[3], width, height, p->linesize[3], 2);
     }else{