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
```


{
    if(coreThread)
    {
        qDebug() << __func__ << ": Stopping thread";
        coreThread->quit();
        coreThread->wait();
        qDebug() << __func__ << ": Stopped thread";
    }
    delete window;
    window = nullptr;
#ifdef ENABLE_WALLET
    delete paymentServer;
    paymentServer = nullptr;
    delete m_wallet_controller;
    m_wallet_controller = nullptr;
#endif
    delete optionsModel;
    optionsModel = nullptr;
    delete platformStyle;
    platformStyle = nullptr;
}
#ifdef ENABLE_WALLET
void BitcoinApplication::createPaymentServer()