ARTEMIS-2183 Useless statement in public synchronized List

Ensures that the returned list returns the refsToAck list instead of
trying to add all items of itself

This closes 2566

artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/RefsOperation.java

@@ -191,7 +191,11 @@ public class RefsOperation extends TransactionOperationAbstract {
    @Override
    public synchronized List<MessageReference> getRelatedMessageReferences() {
       List<MessageReference> listRet = new LinkedList<>();
-      listRet.addAll(listRet);
+
+      if (refsToAck != null && !refsToAck.isEmpty()) {
+         listRet.addAll(refsToAck);
+      }
+
       return listRet;
    }