bn/bn_exp.c: harmonize BN_mod_exp_mont_consttime with negative input.
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 <bc4a6b4cd7a8395c734c8eb5aef04a9c30188c85@oracle.com>
Reviewed-by: Kurt Roeckx <bb87b47479d83cec3c76132206933257ded727b2@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/4676)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl