Ensure ElementData exists before modifying when animating class attribute

Source/core/dom/Element.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *           (C) 2007 Eric Seidel (eric@webkit.org)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1082 matching lines @@
 }
 
 void Element::classAttributeChanged(const AtomicString& newClassString)
 {
     StyleResolver* styleResolver = document().styleResolver();
     bool testShouldInvalidateStyle = inActiveDocument() && styleResolver && styleChangeType() < SubtreeStyleChange;
     bool shouldInvalidateStyle = false;
 
     if (classStringHasClassName(newClassString)) {
         const bool shouldFoldCase = document().inQuirksMode();
-        const SpaceSplitString oldClasses = elementData()->classNames();
+        const SpaceSplitString oldClasses = ensureUniqueElementData()->classNames();
         elementData()->setClass(newClassString, shouldFoldCase);
         const SpaceSplitString& newClasses = elementData()->classNames();
         shouldInvalidateStyle = testShouldInvalidateStyle && checkSelectorForClassChange(oldClasses, newClasses, styleResolver->ensureRuleFeatureSet());
-    } else {
+    } else if (elementData()) {
         const SpaceSplitString& oldClasses = elementData()->classNames();
         shouldInvalidateStyle = testShouldInvalidateStyle && checkSelectorForClassChange(oldClasses, styleResolver->ensureRuleFeatureSet());
         elementData()->clearClass();
     }
 
     if (hasRareData())
         elementRareData()->clearClassListValueForQuirksMode();
 
     if (shouldInvalidateStyle)
         setNeedsStyleRecalc();
@@ skipping 2502 matching lines @@
     // Before doing so, we need to resolve issues in HTMLSelectElement::recalcListItems
     // and RenderMenuList::setText. See also https://bugs.webkit.org/show_bug.cgi?id=88405
     if (hasTagName(optionTag) || hasTagName(optgroupTag))
         return false;
     if (FullscreenElementStack::isActiveFullScreenElement(this))
         return false;
     return true;
 }
 
 } // namespace WebCore