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
```


{
    {
        LOCK(cs_args);
        m_config_args.clear();
        m_config_sections.clear();
    }
    const std::string confPath = GetArg("-conf", BITCOIN_CONF_FILENAME);
    fsbridge::ifstream stream(GetConfigFile(confPath));
    
    if (stream.good()) {
        if (!ReadConfigStream(stream, confPath, error, ignore_invalid_keys)) {
            return false;
        }