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 :-)
#include <algorithm>
#include <assert.h>
#include <future>
#include <boost/algorithm/string/replace.hpp>
static CCriticalSection cs_wallets;
static std::vector<std::shared_ptr<CWallet>> vpwallets GUARDED_BY(cs_wallets);
bool AddWallet(const std::shared_ptr<CWallet>& wallet)
{
    LOCK(cs_wallets);
    assert(wallet);
    std::vector<std::shared_ptr<CWallet>>::const_iterator i = std::find(vpwallets.begin(), vpwallets.end(), wallet);
    if (i != vpwallets.end()) return false;
    vpwallets.push_back(wallet);
    return true;
}
bool RemoveWallet(const std::shared_ptr<CWallet>& wallet)
{
    LOCK(cs_wallets);
    assert(wallet);
    std::vector<std::shared_ptr<CWallet>>::iterator i = std::find(vpwallets.begin(), vpwallets.end(), wallet);
    if (i == vpwallets.end()) return false;
    vpwallets.erase(i);
    return true;
}
bool HasWallets()
{
    LOCK(cs_wallets);
    return !vpwallets.empty();
}
std::vector<std::shared_ptr<CWallet>> GetWallets()