ffv1dec: plane_index is 1 in case of version 4 gray+alpha.

Since version 4, plane_index for the alpha plane is 1 in the case chroma_planes is 0.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ffv1dec.c

@@ -441,7 +441,7 @@ static int decode_slice(AVCodecContext *c, void *arg)
             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);
+            decode_plane(fs, p->data[3] + ps*x + y*p->linesize[3], width, height, p->linesize[3], (f->version >= 4 && !f->chroma_planes) ? 1 : 2);
     } else {
         uint8_t *planes[3] = { p->data[0] + ps * x + y * p->linesize[0],
                                p->data[1] + ps * x + y * p->linesize[1],