Copy topic and message before release the latch, else the receiver co…

…uld read null values

broker/src/test/java/io/moquette/integration/mqtt5/SubscriptionOptionsTest.java

@@ -58,9 +58,9 @@ static class PublishCollector implements IMqttMessageListener {
 
         @Override
         public void messageArrived(String topic, MqttMessage message) throws Exception {
-            latch.countDown();
             receivedTopic = topic;
             receivedMessage = message;
+            latch.countDown();
         }
 
         public String receivedPayload() {