Fix exception handling when using invalid host names upon connect.

Source/MQTTnet/Client/MqttClient.cs

@@ -333,7 +333,7 @@ namespace MQTTnet.Client
 
                 await Task.WhenAll(receiverTask, publishPacketReceiverTask, keepAliveTask).ConfigureAwait(false);
 
-                _publishPacketReceiverQueue.Dispose();
+                _publishPacketReceiverQueue?.Dispose();
             }
             catch (Exception e)
             {

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -71,6 +71,8 @@ namespace MQTTnet.Implementations
 
             await socket.ConnectAsync(_options.Server, _options.GetPort(), cancellationToken).ConfigureAwait(false);
 
+            cancellationToken.ThrowIfCancellationRequested();
+
             var networkStream = socket.GetStream();
 
             if (_options.TlsOptions?.UseTls == true)