OpenSSL::BN#to_i

static VALUE
ossl_bn_to_i(VALUE self)
{
    BIGNUM *bn;
    char *txt;
    VALUE num;

    GetBN(self, bn);

    if (!(txt = BN_bn2hex(bn))) {
        ossl_raise(eBNError, NULL);
    }
    num = rb_cstr_to_inum(txt, 16, Qtrue);
    OPENSSL_free(txt);

    return num;
}