OpenSSL::BN#initialize_copy

static VALUE
ossl_bn_copy(VALUE self, VALUE other)
{
    BIGNUM *bn1, *bn2;

    rb_check_frozen(self);

    if (self == other) return self;

    GetBN(self, bn1);
    bn2 = GetBNPtr(other);

    if (!BN_copy(bn1, bn2)) {
        ossl_raise(eBNError, NULL);
    }
    return self;
}