bitcoind won't start if datadir is specified in the conf file and the user running it has no home dir

tested with debian stretch and bitcoin 0.17.1

```
# sudo -u bitcoin bitcoind -daemon -conf=/usr/local/etc/bitcoin/bitcoin.conf -pid=/run/bitcoind/bitcoind.pid

************************
EXCEPTION: N5boost10filesystem16filesystem_errorE
boost::filesystem::create_directories: Permission denied: "/home/bitcoin"
bitcoin in AppInit()

bitcoind: chainparamsbase.cpp:29: const CBaseChainParams& BaseParams(): Assertion `globalChainBaseParams' failed.
Aborted

# mkhomedir_helper bitcoin

# sudo -u bitcoin bitcoind -daemon -conf=/usr/local/etc/bitcoin/bitcoin.conf -pid=/run/bitcoind/bitcoind.pid
Bitcoin server starting
```


{
    qDebug() << __func__ << ": Initialization result: " << success;
    
    returnValue = success ? EXIT_SUCCESS : EXIT_FAILURE;
    if(success)
    {
        
        qInfo() << "Platform customization:" << platformStyle->getName();
#ifdef ENABLE_WALLET
        m_wallet_controller = new WalletController(m_node, platformStyle, optionsModel, this);
#ifdef ENABLE_BIP70
        PaymentServer::LoadRootCAs();
#endif
        if (paymentServer) {
            paymentServer->setOptionsModel(optionsModel);
#ifdef ENABLE_BIP70
            connect(m_wallet_controller, &WalletController::coinsSent, paymentServer, &PaymentServer::fetchPaymentACK);
#endif
        }
#endif
        clientModel = new ClientModel(m_node, optionsModel);
        window->setClientModel(clientModel);
#ifdef ENABLE_WALLET
        window->setWalletController(m_wallet_controller);