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 :-)
{
    CAmount nTotal = 0;
    {
        LOCK2(cs_main, cs_wallet);
        for (const auto& entry : mapWallet)
        {
            const CWalletTx* pcoin = &entry.second;
            nTotal += pcoin->GetImmatureWatchOnlyCredit();
        }
    }
    return nTotal;
}
CAmount CWallet::GetLegacyBalance(const isminefilter& filter, int minDepth, const std::string* account) const
{
    LOCK2(cs_main, cs_wallet);
    CAmount balance = 0;
    for (const auto& entry : mapWallet) {
        const CWalletTx& wtx = entry.second;
        const int depth = wtx.GetDepthInMainChain();
        if (depth < 0 || !CheckFinalTx(*wtx.tx) || wtx.GetBlocksToMaturity() > 0) {
            continue;
        }
        
        
        CAmount debit = wtx.GetDebit(filter);
        const bool outgoing = debit > 0;
        for (const CTxOut& out : wtx.tx->vout) {