More consistently defend against division by zero with assertions. We

now check that all the modular functions (modpow, modinv, modmul,
bigdivmod) have nonzero moduli, and that modinv also has a nonzero
thing to try to invert.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9987 cda61777-01e9-0310-a592-d414129be87e