8078408: Java version applet hangs with Voice over turned on

Summary: add null check to fix NPE
Reviewed-by: prr, serb, alexsch
Contributed-by: peter.brunet@oracle.com

src/macosx/classes/sun/lwawt/macosx/CAccessibility.java

 /*
- * Copyright (c) 2011, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2011, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -630,7 +630,7 @@ class CAccessibility implements PropertyChangeListener {
 
             if (!allowIgnored) {
                 final AccessibleRole role = context.getAccessibleRole();
-                if (role != null && ignoredRoles.contains(roleKey(role))) {
+                if (role != null && ignoredRoles != null && ignoredRoles.contains(roleKey(role))) {
                     // Get the child's unignored children.
                     _addChildren(child, whichChildren, false, childrenAndRoles);
                 } else {