lavf: rescale duration for chained muxers.

Fix trac ticket #2300 because the duration of the segments
was computed using the timestamp of the last packet plus its
duration using the 1/90000 default time base instead of using
the chained muxer time base.

libavformat/utils.c

@@ -3967,6 +3967,10 @@ int ff_write_chained(AVFormatContext *dst, int dst_stream, AVPacket *pkt,
         local_pkt.dts = av_rescale_q(pkt->dts,
                                      src->streams[pkt->stream_index]->time_base,
                                      dst->streams[dst_stream]->time_base);
+    if (pkt->duration)
+        local_pkt.duration = av_rescale_q(pkt->duration,
+                                          src->streams[pkt->stream_index]->time_base,
+                                          dst->streams[dst_stream]->time_base);
     return av_write_frame(dst, &local_pkt);
 }