lavf/segment: fix DTS inconsistencies with -reset_timestamps

The DTS needs to be resynched against the segment start PTS, or the
resulting DTS may result < PTS.

Reported-by: Owen Jones <riots6@gmail.com>

See thread:
Subject: [FFmpeg-user] pts/dts error using reset_timestamps while splitting a DVD
Date: Sat, 19 Jan 2013 08:58:27 +0000