Invalid characters for a wallet name, e.g. `bitcoin-qt -wallet=你好` used to result in an error like this

<img width="494" alt="schermafbeelding 2018-07-24 om 17 46 37" src="https://user-images.githubusercontent.com/10217/43150553-7bdf598a-8f6a-11e8-8972-44c311a57e56.png">

But now it just crashes:

<img width="509" alt="schermafbeelding 2018-07-24 om 17 43 26" src="https://user-images.githubusercontent.com/10217/43150605-9cf32192-8f6a-11e8-9333-87d074c8dd35.png">

I didn't do a `git bisect`, but it broke somewhere this year :-)
nBytes = CalculateMaximumSignedTxSize(txNew, this);
                if (nBytes < 0) {
                    strFailReason = _("Signing transaction failed");
                    return false;
                }
                nFeeNeeded = GetMinimumFee(*this, nBytes, coin_control, ::mempool, ::feeEstimator, &feeCalc);
                if (feeCalc.reason == FeeReason::FALLBACK && !m_allow_fallback_fee) {
                    
                    strFailReason = _("Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable -fallbackfee.");
                    return false;
                }
                
                
                if (nFeeNeeded < ::minRelayTxFee.GetFee(nBytes))
                {
                    strFailReason = _("Transaction too large for fee policy");
                    return false;
                }
                if (nFeeRet >= nFeeNeeded) {
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    if (nChangePosInOut == -1 && nSubtractFeeFromAmount == 0 && pick_new_inputs) {
                        unsigned int tx_size_with_change = nBytes + coin_selection_params.change_output_size + 2;