H.264: fix grayscale decoding with explicit wpred

Originally committed as revision 26306 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -1371,7 +1371,8 @@ static int pred_weight_table(H264Context *h){
     h->use_weight= 0;
     h->use_weight_chroma= 0;
     h->luma_log2_weight_denom= get_ue_golomb(&s->gb);
-    h->chroma_log2_weight_denom= get_ue_golomb(&s->gb);
+    if(CHROMA)
+        h->chroma_log2_weight_denom= get_ue_golomb(&s->gb);
     luma_def = 1<<h->luma_log2_weight_denom;
     chroma_def = 1<<h->chroma_log2_weight_denom;