mp_exptmod_base_2() uses Montogmery method and does not support even
modulus. Added check.
mp_exptmod_fast() uses Montogmery method when odd not when dr > 0.
Comment updated at call.
Reduce value of zero doesn't work for mp_reduece_2k. Changed setup of
use to check for zero and won't use it. Other methods won't work either.
b18bc86ef8