avformat/mux: Update sidedata in ff_write_chained()

Fixes Ticket4777
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit db91e0ed)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mux.c

@@ -991,6 +991,8 @@ int ff_write_chained(AVFormatContext *dst, int dst_stream, AVPacket *pkt,
     if (interleave) ret = av_interleaved_write_frame(dst, &local_pkt);
     else            ret = av_write_frame(dst, &local_pkt);
     pkt->buf = local_pkt.buf;
+    pkt->side_data       = local_pkt.side_data;
+    pkt->side_data_elems = local_pkt.side_data_elems;
     pkt->destruct = local_pkt.destruct;
     return ret;
 }