BN_mod_exp_mont_consttime: check for zero modulus.

Don't dereference |d| when |top| is zero. Also test that various BIGNUM methods behave correctly on zero/even inputs.

Follow-up to b11980d79a52ec08844f08bea0e66c04b691840b

Reviewed-by: Rich Salz <rsalz@openssl.org>