Prevent negative values on billing report
Prevent negative values on billing report

This was caused by a negative value on the end_time field of a Subtitle
instance.  Some legacy data still has this value to indicate that the subtitle
doesn't have a synced end.

eloquence/unisubs,eloquence/unisubs,ofer43211/unisubs,pculture/unisubs,wevoice/wesub,ofer43211/unisubs,pculture/unisubs,ujdhesa/unisubs,wevoice/wesub,norayr/unisubs,wevoice/wesub,eloquence/unisubs,ReachingOut/unisubs,norayr/unisubs,pculture/unisubs,ujdhesa/unisubs,eloquence/unisubs,ReachingOut/unisubs,ReachingOut/unisubs,ujdhesa/unisubs,ofer43211/unisubs,ujdhesa/unisubs,wevoice/wesub,ReachingOut/unisubs,pculture/unisubs,norayr/unisubs,norayr/unisubs,ofer43211/unisubs