generate_key!

static VALUE
ossl_dh_generate_key(VALUE self)
{
    DH *dh;
    EVP_PKEY *pkey;

    GetPKeyDH(self, pkey);
    dh = pkey->pkey.dh;

    if (!DH_generate_key(dh))
        ossl_raise(eDHError, "Failed to generate key");
    return self;
}