[base] Tighten the overflow check in `FT_MulFix'.

* src/base/ftcalc.c (FT_MulFix) [!FT_LONG64]: Updated.
[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.
Fix Savannah bug #43153.

* src/psaux/psconv.c (PS_Conv_ToFixed): Add protection against
overflow in `divider'.
[base] Tighten the overflow check in `FT_MulDiv'.

* src/base/ftcalc.c (FT_MulDiv) [!FT_LONG64]: Updated.
Merge branch 'master' of git://git.sv.nongnu.org/freetype/freetype2