[base] Tighten the overflow check in `FT_DivFix'. This fixes a 13-year old bug. The original overflow check should have been updated when rounding was introduced into this function (c2cd00443b). * src/base/ftcalc.c (FT_DivFix) [!FT_LONG64]: Updated. * include/freetype.h (FT_DivFix): Updated documentation.