matroska_parse_frame: fix memleak

Fixes CID991856
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -2140,6 +2140,8 @@ static int matroska_parse_frame(MatroskaDemuxContext *matroska,
                                                      AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL,
                                                      additional_size + 8);
         if(side_data == NULL) {
+            av_free_packet(pkt);
+            av_free(pkt);
             return AVERROR(ENOMEM);
         }
         AV_WB64(side_data, additional_id);