lavf: Set the stream time base hint properly for chained muxers

This avoids warnings about using the codec time base as time
base hint.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/segment.c

@@ -74,6 +74,7 @@ static int segment_mux_init(AVFormatContext *s)
             return AVERROR(ENOMEM);
         avcodec_copy_context(st->codec, s->streams[i]->codec);
         st->sample_aspect_ratio = s->streams[i]->sample_aspect_ratio;
+        st->time_base = s->streams[i]->time_base;
     }
 
     return 0;