avformat/dashenc: only write video streams into HLS master playlist

Tool mediastreamvalidator reports error "Variant media_[N].m3u8 is
missing audio group" for audio streams in HLS master playlist. As audio
streams are already listed in audio group, skip them as variant media
streams in master playlist.

libavformat/dashenc.c

@@ -911,8 +911,10 @@ static int write_manifest(AVFormatContext *s, int final)
             OutputStream *os = &c->streams[i];
             char *agroup = NULL;
             int stream_bitrate = st->codecpar->bit_rate + os->muxer_overhead;
+            if (st->codecpar->codec_type != AVMEDIA_TYPE_VIDEO)
+                continue;
             av_strlcpy(codec_str, os->codec_str, sizeof(codec_str));
-            if ((st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) && max_audio_bitrate) {
+            if (max_audio_bitrate) {
                 agroup = (char *)audio_group;
                 stream_bitrate += max_audio_bitrate;
                 av_strlcat(codec_str, ",", sizeof(codec_str));