lavf/matroskaenc: Do not needlessly allocate memory for cuepoints.

Fixes ticket #4690.

libavformat/matroskaenc.c

@@ -1772,7 +1772,7 @@ static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt, int add_
 
     if (codec->codec_type != AVMEDIA_TYPE_SUBTITLE) {
         mkv_write_block(s, pb, MATROSKA_ID_SIMPLEBLOCK, pkt, keyframe);
-        if (codec->codec_type == AVMEDIA_TYPE_VIDEO && keyframe || add_cue) {
+        if (s->pb->seekable && (codec->codec_type == AVMEDIA_TYPE_VIDEO && keyframe || add_cue)) {
             ret = mkv_add_cuepoint(mkv->cues, pkt->stream_index, dash_tracknum, ts, mkv->cluster_pos, relative_packet_pos, -1);
             if (ret < 0) return ret;
         }
@@ -1792,11 +1792,13 @@ static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt, int add_
         end_ebml_master(pb, blockgroup);
     }
 
+    if (s->pb->seekable) {
     ret = mkv_add_cuepoint(mkv->cues, pkt->stream_index, dash_tracknum, ts,
                            mkv->cluster_pos, relative_packet_pos, duration);
     if (ret < 0)
         return ret;
     }
+    }
 
     mkv->duration = FFMAX(mkv->duration, ts + duration);
     return 0;