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.

{
    ENTER_CRITICAL_SECTION(cs_vSend);
    assert(ssSend.size() == 0);
    ssSend << CMessageHeader(Params().MessageStart(), pszCommand, 0);
    LogPrint("net", "sending: %s ", SanitizeString(pszCommand));
}
void CNode::AbortMessage() UNLOCK_FUNCTION(cs_vSend)
{
    ssSend.clear();
    LEAVE_CRITICAL_SECTION(cs_vSend);
    LogPrint("net", "(aborted)\n");
}
void CNode::EndMessage(const char* pszCommand) UNLOCK_FUNCTION(cs_vSend)