A stopped TcpListener will NRE during AllowNatTraversal

[eerhardt]

Execute the following code on .NET 5:

var listener = new TcpListener(IPAddress.Any, 0);
listener.Start();
listener.Stop();
listener.AllowNatTraversal(true);

It will throw a NRE on line 121:

runtime/src/libraries/System.Net.Sockets/src/System/Net/Sockets/TCPListener.cs

Lines 114 to 121 in e41651e

	public void AllowNatTraversal(bool allowed)
	{
	if (_active)
	{
	throw new InvalidOperationException(SR.net_tcplistener_mustbestopped);
	}
	_serverSocket.SetIPProtectionLevel(allowed ? IPProtectionLevel.Unrestricted : IPProtectionLevel.EdgeRestricted);

This appears to be a regression in 5.0 caused by dotnet/corefx#41547. In previous versions, _serverSocket would still be valid after calling .Stop().

Note that AllowNatTraversal can only be called on a stopped listener, so with this bug it means you can only call AllowNatTraversal immediately after creating the listener and before calling Start.

/cc @alnikola