avcodec/h264: reduce noisiness of "mmco: unref short failure"

Do not consider it an error if we have no frames and should discard one.
This condition can easily happen when decoding is started from an I frame

Fixes Ticket2811
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_refs.c

@@ -582,7 +582,7 @@ int ff_h264_execute_ref_pic_marking(H264Context *h, MMCO *mmco, int mmco_count)
                 if (mmco[i].opcode != MMCO_SHORT2LONG ||
                     !h->long_ref[mmco[i].long_arg]    ||
                     h->long_ref[mmco[i].long_arg]->frame_num != frame_num) {
-                    av_log(h->avctx, AV_LOG_ERROR, "mmco: unref short failure\n");
+                    av_log(h->avctx, h->short_ref_count ? AV_LOG_ERROR : AV_LOG_DEBUG, "mmco: unref short failure\n");
                     err = AVERROR_INVALIDDATA;
                 }
                 continue;