`AddressAlreadyInUse` when using `WithDefaultEndPoint` but server starts anyway.

[timothyparez]

Describe the bug

Upgrading from 3.x to the latest 4.x, I wanted to create a server without encryption. (for local use only)

Original 3.x code:

var options = new MqttServerOptionsBuilder().WithConnectionBacklog(10)
                            .WithDefaultEndpointPort(32700)
                            .WithDefaultEndpointBoundIPAddress(System.Net.IPAddress.Loopback)
                            .WithApplicationMessageInterceptor(HandleApplicationMessage)
                            .WithSubscriptionInterceptor(HandleSubscription)
                            .Build();

New 4.x code:

var options = new MqttServerOptionsBuilder().WithConnectionBacklog(10)                                                        
                            .WithoutEncryptedEndpoint()         
                            .WithDefaultEndpoint() //To enable endpoint without encryption                                            
                            .WithDefaultEndpointPort(32700)
                            .WithDefaultEndpointBoundIPAddress(System.Net.IPAddress.Loopback)
                            .Build();

As soon as await server.StartAsync(); is called the 4.x version throws the following exception:

Failed to start the broker: System.Net.Sockets.SocketException (0x80004005): Address already in use

However, the server is still started and clients can connect to it.
This is the only instance of the server and this code is only called once. If await server.StartAsync(); is commented out, the server is not started (obviously). There are no other processes on the system using that port.

Which component is your bug related to?

• Server

To Reproduce

Steps to reproduce the behavior:

1. Using this version of MQTTnet '4.1.3.436'.
2. Run this code: The 4.x code above
3. With these arguments 'WithDefaultEndpoint'
4. See error: SocketException (but server starts anyway)

Expected behavior

Expected it to start the server without an exception since the port is not in use

Additional context / logging

• This is running on an isolated embedded system.
• It works perfectly with 3.x
• After catching the exception, clients can connect normally
• Clients run on the embedded system. There is no outside communication
• Nothing else is using the port
• Changing the port to a different value does not resolve the issue
• OS = Android (Custom BSP, completely under our control, so no restrictions)
• Framework = Mono