Fix segfault reported by ClusterFuzz.

Source/core/dom/FullscreenElementStack.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, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 43 matching lines @@
         if (!owner->hasAttribute(allowfullscreenAttr))
             return false;
     }
     return true;
 }
 
 static bool fullscreenIsSupported(const Document& document)
 {
     // Fullscreen is supported if there is no previously-established user preference,
     // security risk, or platform limitation.
-    return document.settings()->fullscreenSupported();
+    return !document.settings() || document.settings()->fullscreenSupported();
 }
 
 static bool fullscreenIsSupported(const Document& document, const Element& element)
 {
-    if (document.settings()->disallowFullscreenForNonMediaElements() && !isHTMLMediaElement(element))
+    if (!document.settings() || (document.settings()->disallowFullscreenForNonMediaElements() && !isHTMLMediaElement(element)))
         return false;
     return fullscreenIsSupported(document);
 }
 
 static PassRefPtrWillBeRawPtr<Event> createEvent(const AtomicString& type, EventTarget& target)
 {
     RefPtrWillBeRawPtr<Event> event = Event::createBubble(type);
     event->setTarget(&target);
     return event;
 }
@@ skipping 510 matching lines @@
 
 void FullscreenElementStack::trace(Visitor* visitor)
 {
     visitor->trace(m_fullScreenElement);
     visitor->trace(m_fullScreenElementStack);
     visitor->trace(m_eventQueue);
     DocumentSupplement::trace(visitor);
 }
 
 } // namespace blink