Fix parsing of MQTT messages that don't contain a timestamp

CollectAgent/collectagent.cpp

@@ -135,7 +135,7 @@ void mqttCallback(SimpleMQTTMessage *msg)
         payload = &buf;
         payload->value = *((uint64_t*)msg->getPayload());
         payload->timestamp = Messaging::calculateTimestamp();
-        len = 2;
+        len = sizeof(uint64_t) * 2;
       }
       //...otherwise it just retrieves it from the MQTT message payload.
       else if((len%sizeof(mqttPayload)==0) && (len>0)){