Fix null pointer dereference in ChromeClientImpl::openBeforeUnloadConfirmPanelDelegate.

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 2538 matching lines @@
         return false;
 
     RefPtrWillBeRawPtr<Document> protect(this);
 
     RefPtrWillBeRawPtr<BeforeUnloadEvent> beforeUnloadEvent = BeforeUnloadEvent::create();
     m_loadEventProgress = BeforeUnloadEventInProgress;
     m_domWindow->dispatchEvent(beforeUnloadEvent.get(), this);
     m_loadEventProgress = BeforeUnloadEventCompleted;
     if (!beforeUnloadEvent->defaultPrevented())
         defaultEventHandler(beforeUnloadEvent.get());
-    if (beforeUnloadEvent->returnValue().isNull())
+    if (!frame() || beforeUnloadEvent->returnValue().isNull())
         return true;
 
     if (didAllowNavigation) {
         addConsoleMessage(ConsoleMessage::create(JSMessageSource, ErrorMessageLevel, "Blocked attempt to show multiple 'beforeunload' confirmation panels for a single navigation."));
         return true;
     }
 
     String text = beforeUnloadEvent->returnValue();
     if (chromeClient.openBeforeUnloadConfirmPanel(text, m_frame)) {
         didAllowNavigation = true;
@@ skipping 3140 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif