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

Source/core/rendering/RenderObject.cpp

Index: Source/core/rendering/RenderObject.cpp
diff --git a/Source/core/rendering/RenderObject.cpp b/Source/core/rendering/RenderObject.cpp
index 22f2163d4574f6059c2da888d0f3fd37a377a01d..b0f243eb49648f81f1684d0e232728cd42c442e4 100644
--- a/Source/core/rendering/RenderObject.cpp
+++ b/Source/core/rendering/RenderObject.cpp
@@ -529,10 +529,9 @@ RenderLayer* RenderObject::enclosingLayer() const
         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;
 }