avformat/segment: populate empty outer stream extradata from packet

At present, if the outer stream extradata is empty but first packet
has extradata as a side data element, then only the first segment's
muxer instance may be able to extract this side data and use it.
For all other segments, extradata in packet side data could be missing
and generated segments may be invalid or unplayable in some apps
e.g. for an ADTS AAC stream segmented to MP4, the adtstoasc BSF will
add extradata to the first packet. The MOV muxer for the first segment
will add this to codecpar for the inner stream and write
Decoder Specific Information within the esds box. For other segments,
their esds' will not have this decSpecificInfo and they can't be opened
in Quicktime player or by services like nginx-vod-module.

libavformat/segment.c

@@ -859,6 +859,20 @@ static int seg_write_packet(AVFormatContext *s, AVPacket *pkt)
     if (!seg->avf || !seg->avf->pb)
         return AVERROR(EINVAL);
 
+    if (!st->codecpar->extradata_size) {
+        int pkt_extradata_size = 0;
+        uint8_t *pkt_extradata = av_packet_get_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA, &pkt_extradata_size);
+        if (pkt_extradata && pkt_extradata_size > 0) {
+            ret = ff_alloc_extradata(st->codecpar, pkt_extradata_size);
+            if (ret < 0) {
+                av_log(s, AV_LOG_WARNING, "Unable to add extradata to stream. Output segments may be invalid.\n");
+                goto calc_times;
+            }
+            memcpy(st->codecpar->extradata, pkt_extradata, pkt_extradata_size);
+            st->codecpar->extradata_size = pkt_extradata_size;
+        }
+    }
+
 calc_times:
     if (seg->times) {
         end_pts = seg->segment_count < seg->nb_times ?