matroskadec: fix memleak of pkt_data

Fixes: CID1026767
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -2264,7 +2264,8 @@ static int matroska_parse_frame(MatroskaDemuxContext *matroska,
     /* XXX: prevent data copy... */
     if (av_new_packet(pkt, pkt_size + offset) < 0) {
         av_free(pkt);
-        return AVERROR(ENOMEM);
+        res = AVERROR(ENOMEM);
+        goto fail;
     }
 
     if (st->codec->codec_id == AV_CODEC_ID_PRORES) {