Avoid a special case in Barrett reduction for x < mod
Avoid a special case in Barrett reduction for x < mod

This would have prevented CVE-2018-12435

randombit/botan,randombit/botan,webmaster128/botan,randombit/botan,webmaster128/botan,webmaster128/botan,webmaster128/botan,webmaster128/botan,randombit/botan,randombit/botan