fontconfig: fix misplaced overflow check
fontconfig: fix misplaced overflow check

This allowed writes to one past the end of `families` and `fullnames`.

MrSmile/libass,xbmc/libass,ShiftMediaProject/libass,libass/libass,MrSmile/libass,astiob/libass,libass/libass,ShiftMediaProject/libass,xbmc/libass,astiob/libass