avcodec/vc1dec: print debug message if a b frame without reference is skiped

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

libavcodec/vc1dec.c

@@ -5997,6 +5997,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
 
     /* skip B-frames if we don't have reference frames */
     if (s->last_picture_ptr == NULL && (s->pict_type == AV_PICTURE_TYPE_B || s->droppable)) {
+        av_log(v->s.avctx, AV_LOG_DEBUG, "Skiping B frame without reference frames\n");
         goto end;
     }
     if ((avctx->skip_frame >= AVDISCARD_NONREF && s->pict_type == AV_PICTURE_TYPE_B) ||