Prevent NPEx

mq/main/comm-util/src/main/java/com/sun/messaging/jmq/util/lists/FifoSet.java

@@ -135,7 +135,7 @@ public E next() {
             if (current == null && initialPass) {
                 current = first_entry;
                 initialPass = false;
-            } else {
+            } else if (current != null) {
                 current = current.getNext();
             }
 

mq/main/comm-util/src/test/java/com/sun/messaging/jmq/util/lists/FifoSetIteratorTest.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2020 Contributors to the Eclipse Foundation
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0, which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * This Source Code may also be made available under the following Secondary
+ * Licenses when the conditions for such availability set forth in the
+ * Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
+ * version 2 with the GNU Classpath Exception, which is available at
+ * https://www.gnu.org/software/classpath/license.html.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
+ */
+
+package com.sun.messaging.jmq.util.lists;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.Set;
+import java.util.TreeSet;
+
+import org.junit.jupiter.api.Test;
+
+class FifoSetIteratorTest {
+
+    @Test
+    void consecutiveNextsOnEmptySetShouldThrowNoSuchElementException() {
+        consecutiveNextsOnEmptySetShouldThrowNoSuchElementException(new FifoSet());
+    }
+
+    @Test
+    void consecutiveNextsOnStandardSetThrowNoSuchElementException() {
+        consecutiveNextsOnEmptySetShouldThrowNoSuchElementException(new TreeSet());
+    }
+
+    private void consecutiveNextsOnEmptySetShouldThrowNoSuchElementException(Set s) {
+        Iterator iterator = s.iterator();
+        assertThrows(NoSuchElementException.class, () -> iterator.next());
+        assertThrows(NoSuchElementException.class, () -> iterator.next());
+    }
+}