Avoid repeated size checks when setting words in NIST reduction
Avoid repeated size checks when setting words in NIST reduction

This is a tiny thing but it saves over 100K cycles for P-384 ECDSA

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