lavf/mpegtsenc: Only fail aac muxing if the first frame is invalid.

Fixes ticket #3957.

libavformat/mpegtsenc.c

@@ -1313,9 +1313,9 @@ static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
             if (!ts_st->amux) {
                 av_log(s, AV_LOG_ERROR, "AAC bitstream not in ADTS format "
                                         "and extradata missing\n");
-                return AVERROR_INVALIDDATA;
-            }
-
+                if (!st->nb_frames)
+                    return AVERROR_INVALIDDATA;
+            } else {
             av_init_packet(&pkt2);
             pkt2.data = pkt->data;
             pkt2.size = pkt->size;
@@ -1334,6 +1334,7 @@ static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
             size            = avio_close_dyn_buf(ts_st->amux->pb, &data);
             ts_st->amux->pb = NULL;
             buf             = data;
+            }
         }
     } else if (st->codec->codec_id == AV_CODEC_ID_HEVC) {
         int ret = check_hevc_startcode(s, st, pkt);