Fix a ** 0 mod 1 = 0 for real this time.

Commit 2b0180c3

 attempted to do this but
only hit one of many BN_mod_exp codepaths. Fix remaining variants and add
a test for each method.

Thanks to Hanno Boeck for reporting this issue.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Dr. Stephen Henson <steve@openssl.org>