avformat/demux: extract extradata from packets when context update is requested

If the demuxer doesn't set extradata in the stream's codecpar, a
need_context_update request will delete the previously extracted extradata in
the stream's internal AVCodecContext.
As we can't ensure the old extradata is valid for the stream in its post
context update request state, try to get extradata from the new packet instead
of attempting to preserve the old in some form.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/demux.c

@@ -1319,6 +1319,8 @@ fail:
     return ret;
 }
 
+static int extract_extradata(FFFormatContext *si, AVStream *st, const AVPacket *pkt);
+
 static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
 {
     FFFormatContext *const si = ffformatcontext(s);
@@ -1373,6 +1375,16 @@ static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
                 return ret;
             }
 
+            if (!sti->avctx->extradata) {
+                sti->extract_extradata.inited = 0;
+
+                ret = extract_extradata(si, st, pkt);
+                if (ret < 0) {
+                    av_packet_unref(pkt);
+                    return ret;
+                }
+            }
+
             sti->codec_desc = avcodec_descriptor_get(sti->avctx->codec_id);
 
             sti->need_context_update = 0;
@@ -2427,6 +2439,7 @@ static int extract_extradata_init(AVStream *st)
     if (!ret)
         goto finish;
 
+    av_bsf_free(&sti->extract_extradata.bsf);
     ret = av_bsf_alloc(f, &sti->extract_extradata.bsf);
     if (ret < 0)
         return ret;