Warn about unimplemented H264 feature.

Originally committed as revision 13542 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -4112,6 +4112,8 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
     if(h->slice_type == FF_P_TYPE || h->slice_type == FF_SP_TYPE || h->slice_type == FF_B_TYPE){
         if(h->slice_type == FF_B_TYPE){
             h->direct_spatial_mv_pred= get_bits1(&s->gb);
+            if(FIELD_PICTURE && h->direct_spatial_mv_pred)
+                av_log(h->s.avctx, AV_LOG_ERROR, "PAFF + spatial direct mode is not implemented\n");
         }
         num_ref_idx_active_override_flag= get_bits1(&s->gb);