Current bitcoin-master on testnet, `maxconnections=8` in the config file. Running in VM that doesn't have public IP and is not reachable from the Internet.

Wait until you have 8 peers.

```
telnet localhost 18333
```

It will result in:

```
Assertion failed: (nMaxInbound > 0), function AcceptConnection, file net.cpp, line 973.
```

First found in https://github.com/bitcoin/bitcoin/pull/8996, but can be reproduced on master.

CSubNet subNet(addr);
    return Unban(subNet);
}
bool CConnman::Unban(const CSubNet &subNet) {
    {
        LOCK(cs_setBanned);
        if (!setBanned.erase(subNet))
            return false;
        setBannedIsDirty = true;
    }
    if(clientInterface)
        clientInterface->BannedListChanged();
    DumpBanlist(); 
    return true;
}
void CConnman::GetBanned(banmap_t &banMap)
{
    LOCK(cs_setBanned);
    banMap = setBanned; 
}
void CConnman::SetBanned(const banmap_t &banMap)
{
    LOCK(cs_setBanned);
    setBanned = banMap;
    setBannedIsDirty = true;
}
void CConnman::SweepBanned()