Replace invaled timestamp flag "-1" with AV_NOPTS
-1 is not a good value as a flag for invalid timestamps.
There are cases where small negative timestamps are useful.
So this eliminates a potential ambiguity.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6001 b64f7644-9d1e-0410-96f1-a4d463321fa5