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 :-)
masterKey.SetSeed(seed.begin(), seed.size());
    
    
    masterKey.Derive(accountKey, BIP32_HARDENED_KEY_LIMIT);
    
    assert(internal ? CanSupportFeature(FEATURE_HD_SPLIT) : true);
    accountKey.Derive(chainChildKey, BIP32_HARDENED_KEY_LIMIT+(internal ? 1 : 0));
    
    do {
        
        
        
        if (internal) {
            chainChildKey.Derive(childKey, hdChain.nInternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
            metadata.hdKeypath = "m/0'/1'/" + std::to_string(hdChain.nInternalChainCounter) + "'";
            hdChain.nInternalChainCounter++;
        }
        else {
            chainChildKey.Derive(childKey, hdChain.nExternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
            metadata.hdKeypath = "m/0'/0'/" + std::to_string(hdChain.nExternalChainCounter) + "'";
            hdChain.nExternalChainCounter++;
        }
    } while (HaveKey(childKey.key.GetPubKey().GetID()));
    secret = childKey.key;
    metadata.hd_seed_id = hdChain.seed_id;
    
    if (!batch.WriteHDChain(hdChain))
        throw std::runtime_error(std::string(__func__) + ": Writing HD chain model failed");
}
bool CWallet::AddKeyPubKeyWithDB(WalletBatch &batch, const CKey& secret, const CPubKey &pubkey)