decccsub: fix timestamp of "clear" subtitles

This broke CC passthru and is probably the cause of
https://forum.handbrake.fr/viewtopic.php?f=11&t=29835

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6143 b64f7644-9d1e-0410-96f1-a4d463321fa5