fix: Prevent an exception in UTP when host disconnects (#1447)

The code was invoking the disconnect transport event callback on the
manager before doing some checks on UTP's NetworkConnection. But on a
client, that callback ends up calling Shutdown on the transport. This
disposes of the driver so the checks that follow end up throwing an
exception. The fix was to reorder the checks before the callback.

Also fix the logic that checked if there was a connection failure. It
was checking if the state of the connection was Connecting, but that's
never the case after a Disconnect event (the state will always be
Disconnected).

Author: simon-lemay-unity

com.unity.netcode.adapter.utp/CHANGELOG.md

@@ -10,6 +10,7 @@ All notable changes to this package will be documented in this file. The format
 ### Fixed
 
 - Fixed packet overflow errors when sending payloads too close to the MTU (was mostly visible when using Relay).
+- Don't throw an exception when the host disconnects (issue 1439 on GitHub).
 
 ## [1.0.0-pre.3] - 2021-10-22
 

com.unity.netcode.adapter.utp/Runtime/UnityTransport.cs

@@ -435,20 +435,22 @@ private bool ProcessEvent()
                     }
                 case TransportNetworkEvent.Type.Disconnect:
                     {
-                        InvokeOnTransportEvent(NetcodeNetworkEvent.Disconnect,
-                            ParseClientId(networkConnection),
-                            default(ArraySegment<byte>),
-                            Time.realtimeSinceStartup);
-
                         if (m_ServerConnection.IsCreated)
                         {
                             m_ServerConnection = default;
-                            if (m_Driver.GetConnectionState(m_ServerConnection) == NetworkConnection.State.Connecting)
+
+                            var reason = reader.ReadByte();
+                            if (reason == (byte)Networking.Transport.Error.DisconnectReason.MaxConnectionAttempts)
                             {
                                 Debug.LogError("Client failed to connect to server");
                             }
                         }
 
+                        InvokeOnTransportEvent(NetcodeNetworkEvent.Disconnect,
+                            ParseClientId(networkConnection),
+                            default(ArraySegment<byte>),
+                            Time.realtimeSinceStartup);
+
                         m_State = State.Disconnected;
                         return true;
                     }