avcodec/h264_mb: fix grayscale only decoding with weighted prediction

Fixes Ticket3412
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_mb.c

@@ -420,10 +420,12 @@ static av_always_inline void mc_part_weighted(H264Context *h, int n, int square,
             int weight1 = 64 - weight0;
             luma_weight_avg(dest_y, tmp_y, h->mb_linesize,
                             height, 5, weight0, weight1, 0);
+            if (!CONFIG_GRAY || !(h->flags & CODEC_FLAG_GRAY)) {
                 chroma_weight_avg(dest_cb, tmp_cb, h->mb_uvlinesize,
                                   chroma_height, 5, weight0, weight1, 0);
                 chroma_weight_avg(dest_cr, tmp_cr, h->mb_uvlinesize,
                                   chroma_height, 5, weight0, weight1, 0);
+            }
         } else {
             luma_weight_avg(dest_y, tmp_y, h->mb_linesize, height,
                             h->luma_log2_weight_denom,
@@ -431,6 +433,7 @@ static av_always_inline void mc_part_weighted(H264Context *h, int n, int square,
                             h->luma_weight[refn1][1][0],
                             h->luma_weight[refn0][0][1] +
                             h->luma_weight[refn1][1][1]);
+            if (!CONFIG_GRAY || !(h->flags & CODEC_FLAG_GRAY)) {
                 chroma_weight_avg(dest_cb, tmp_cb, h->mb_uvlinesize, chroma_height,
                                   h->chroma_log2_weight_denom,
                                   h->chroma_weight[refn0][0][0][0],
@@ -444,6 +447,7 @@ static av_always_inline void mc_part_weighted(H264Context *h, int n, int square,
                                   h->chroma_weight[refn0][0][1][1] +
                                   h->chroma_weight[refn1][1][1][1]);
             }
+        }
     } else {
         int list     = list1 ? 1 : 0;
         int refn     = h->ref_cache[list][scan8[n]];
@@ -456,6 +460,7 @@ static av_always_inline void mc_part_weighted(H264Context *h, int n, int square,
                        h->luma_log2_weight_denom,
                        h->luma_weight[refn][list][0],
                        h->luma_weight[refn][list][1]);
+        if (!CONFIG_GRAY || !(h->flags & CODEC_FLAG_GRAY)) {
             if (h->use_weight_chroma) {
                 chroma_weight_op(dest_cb, h->mb_uvlinesize, chroma_height,
                                  h->chroma_log2_weight_denom,
@@ -467,6 +472,7 @@ static av_always_inline void mc_part_weighted(H264Context *h, int n, int square,
                                  h->chroma_weight[refn][list][1][1]);
             }
         }
+    }
 }
 
 static av_always_inline void prefetch_motion(H264Context *h, int list,