avformat/matroskadec: fix resource leak

Fixes Coverity CID: 1405453

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/matroskadec.c

@@ -3893,6 +3893,7 @@ static int webm_dash_manifest_cues(AVFormatContext *s, int64_t init_range)
                            "%" PRId64, s->streams[0]->index_entries[i].timestamp);
         if (ret <= 0 || (ret == 20 && i ==  s->streams[0]->nb_index_entries - 1)) {
             av_log(s, AV_LOG_ERROR, "timestamp too long.\n");
+            av_free(buf);
             return AVERROR_INVALIDDATA;
         }
         end += ret;