avformat/matroskaenc: Check that Cluster has been opened

before setting the field indicating that a Cluster has been opened.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -2311,10 +2311,10 @@ static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt, int add_
     }
 
     if (mkv->cluster_pos == -1) {
-        mkv->cluster_pos = avio_tell(s->pb);
         ret = start_ebml_master_crc32(&mkv->cluster_bc, mkv);
         if (ret < 0)
             return ret;
+        mkv->cluster_pos = avio_tell(s->pb);
         put_ebml_uint(mkv->cluster_bc, MATROSKA_ID_CLUSTERTIMECODE, FFMAX(0, ts));
         mkv->cluster_pts = FFMAX(0, ts);
         av_log(s, AV_LOG_DEBUG,