Don't call RenderObject::enclosingLayer on a null pointer

Source/core/rendering/RenderObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 511 matching lines @@
 
     return 0;
 }
 
 RenderLayer* RenderObject::enclosingLayer() const
 {
     for (const RenderObject* current = this; current; current = current->parent()) {
         if (current->hasLayer())
             return toRenderLayerModelObject(current)->layer();
     }
-    // This line of code should be unreachable. However, if |this| is zero, we
-    // can reach this line. Of course, it's undefined behavior to call a member
-    // function on a zero pointer, but that doesn't stop
-    // FrameView::paintContents. :)
+    // FIXME: We should remove the one caller that triggers this case and make
+    // this function return a reference.
+    ASSERT(!m_parent && !isRenderView());
     return 0;
 }
 
 bool RenderObject::scrollRectToVisible(const LayoutRect& rect, const ScrollAlignment& alignX, const ScrollAlignment& alignY)
 {
     RenderBox* enclosingBox = this->enclosingBox();
     if (!enclosingBox)
         return false;
 
     enclosingBox->scrollRectToVisible(rect, alignX, alignY);
@@ skipping 2801 matching lines @@
 {
     if (object1) {
         const WebCore::RenderObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showRenderTreeAndMark(object1, "*", object2, "-", 0);
     }
 }
 
 #endif