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 :-)
{
    assert(!IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS));
    CKey key;
    key.MakeNewKey(true);
    return DeriveNewSeed(key);
}
CPubKey CWallet::DeriveNewSeed(const CKey& key)
{
    int64_t nCreationTime = GetTime();
    CKeyMetadata metadata(nCreationTime);
    
    CPubKey seed = key.GetPubKey();
    assert(key.VerifyPubKey(seed));
    
    metadata.hdKeypath     = "s";
    metadata.hd_seed_id = seed.GetID();
    {
        LOCK(cs_wallet);
        
        mapKeyMetadata[seed.GetID()] = metadata;
        
        if (!AddKeyPubKey(key, seed))
            throw std::runtime_error(std::string(__func__) + ": AddKeyPubKey failed");
    }
    return seed;
}
void CWallet::SetHDSeed(const CPubKey& seed)