libhb: Fix CC processing for packets that have no PTS

It is not required that every input frame have a PTS, but we were only
creating tags for frames that have a PTS.  If a frame with no PTS happens
to have CC data, we would mistakenly drop the previous CC data and log
a "tag botch" error.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5217 b64f7644-9d1e-0410-96f1-a4d463321fa5