Fix BN_gcd errors for some curves

Those even order that do not play nicely with Montgomery arithmetic

(back-ported from commit 3a6a4a93

)

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6810)