Set gray (128) U/V planes for chroma-less samples. Fixes two fate samples

when played with -flags emu_edge.
(cherry picked from commit 8bcfe7f7fd)

libavcodec/h264_cabac.c

@@ -1373,6 +1373,8 @@ decode_intra_mb:
             pred_mode= ff_h264_check_intra_pred_mode( h, pred_mode );
             if( pred_mode < 0 ) return -1;
             h->chroma_pred_mode= pred_mode;
+        } else {
+            h->chroma_pred_mode= DC_128_PRED8x8;
         }
     } else if( partition_count == 4 ) {
         int i, j, sub_partition_count[4], list, ref[2][4];

libavcodec/h264_cavlc.c

@@ -665,6 +665,8 @@ decode_intra_mb:
             if(pred_mode < 0)
                 return -1;
             h->chroma_pred_mode= pred_mode;
+        } else {
+            h->chroma_pred_mode = DC_128_PRED8x8;
         }
     }else if(partition_count==4){
         int i, j, sub_partition_count[4], list, ref[2][4];