Long password or long usernames breaks auth_login

[interfector18]

Some email providers like brevo have really long passwords (90 characters), and auth_login breaks because it only get's the first 78 characters from base64

void smtp::auth_login(const string& username, const string& password)
{
    dlg_->send("AUTH LOGIN");
    string line = dlg_->receive();
    tuple<int, bool, string> tokens = parse_line(line);
    if (std::get<1>(tokens) && !positive_intermediate(std::get<0>(tokens)))
        throw smtp_error("Authentication rejection.", std::get<2>(tokens));

    // TODO: Use static encode from the Base64 codec.
    base64 b64(static_cast<string::size_type>(codec::line_len_policy_t::RECOMMENDED), static_cast<string::size_type>(codec::line_len_policy_t::RECOMMENDED));
    auto user_v = b64.encode(username);
    string cmd = user_v.empty() ? "" : user_v[0];```