bn/bn_exp.c: harmonize BN_mod_exp_mont_consttime with negative input.

All exponentiation subroutines but BN_mod_exp_mont_consttime produce
non-negative result for negative input, which is confusing for fuzzer.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/4676)