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
```


{
#ifdef WIN32
    util::WinCmdLineArgs winArgs;
    std::tie(argc, argv) = winArgs.get();
#endif
    SetupEnvironment();
    RandomInit();
    try {
        if (!WalletAppInit(argc, argv)) return EXIT_FAILURE;
    } catch (const std::exception& e) {
        PrintExceptionContinue(&e, "WalletAppInit()");
        return EXIT_FAILURE;
    } catch (...) {
        PrintExceptionContinue(nullptr, "WalletAppInit()");