CPU Leak if you specify Username/password

Hi,

Im using M2MQTT from https://www.nuget.org/packages/M2Mqtt/ IF i create a client and specify a user name the CPU usgae goes up by 25% and stays there even if the connection fails due to wrong credentials. And next call to connect will increment the CPU usage by 25%

So if you have multiple thread connecting to MQTT server, after a couple of calls the CPU reached 100% and crashes. Following code can reproduce the issue, add it to a Console application .Net 4.5 or greater:

            Console.WriteLine("start?");
            Console.Read();
            Console.WriteLine("starting, please monitor the CPU usage.");
            List < MqttClient > l = new List<MqttClient>();
            for (int i = 0; i < 10; i++)
            {
                var client = new MqttClient(
                "test.mosquitto.org"
                , 1883
                , false
                , null
                , null
                , new MqttSslProtocols());
                client.MqttMsgPublishReceived += Client_MqttMsgPublishReceived;
                try
                {
                    client.Connect(Guid.NewGuid().ToString(), "ss", null);//intentionally making it fail with wrong credentials
                }
                catch(Exception ex) { Console.WriteLine("Error");  }
                l.Add(client);
            }            

            Console.WriteLine("Exit?");
            Console.Read();```

[GioviQ]

@ngducnghia you can even just commit from Visual Studio.

[ngducnghia]

@GioviQ I forked and submit PR. https://github.com/eclipse/paho.mqtt.m2mqtt/pull/79 I see the code in gnatmq client already have a check for exception. I added another check so if returnCode != CONN_ACCEPTED, it also set isRunning to false to exit the loop in ReceiveThread method.