openssl: BN_mod_exp may produce incorrect results on x86_64

There is a carry propagating bug in the x8664 Montgomery squaring procedure in OpenSSL 1.0.2 before 1.0.2k and 1.1.0 before 1.1.0d. No EC algorithms are affected. Analysis suggests that attacks against RSA and DSA as a result of this defect would be very difficult to perform and are not believed...