rtpenc_chain: Pass the initial time_base hint on to the chained muxer

In practice this hint is ignored - the rtp muxer always overwrites
the stream time base without taking the hint into account. But as
a general practice this is the correct way to pass a time base hint
on to a chained muxer.

This avoids warnings about using the codec time base as hint
being deprecated.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc_chain.c

@@ -73,6 +73,7 @@ int ff_rtp_chain_mux_open(AVFormatContext **out, AVFormatContext *s,
     rtpctx->start_time_realtime = s->start_time_realtime;
 
     avcodec_copy_context(rtpctx->streams[0]->codec, st->codec);
+    rtpctx->streams[0]->time_base = st->time_base;
 
     if (handle) {
         ret = ffio_fdopen(&rtpctx->pb, handle);