Check for null selectors when looping in findBestRuleSetAndAdd

Source/core/css/CSSSelector.h

 /*
  * Copyright (C) 1999-2003 Lars Knoll (knoll@kde.org)
  *               1999 Waldo Bastian (bastian@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2008, 2009, 2010, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 270 matching lines @@
         bool matchesPseudoElement() const;
         bool isUnknownPseudoElement() const;
         bool isCustomPseudoElement() const;
         bool isDirectAdjacentSelector() const { return m_relation == DirectAdjacent; }
         bool isSiblingSelector() const;
         bool isAttributeSelector() const;
         bool isContentPseudoElement() const;
         bool isShadowPseudoElement() const;
         bool isHostPseudoClass() const;
 
-        // FIXME: selectors with no tagHistory() get a relation() of Descendant. It should instead be
+        // FIXME: selectors with no tagHistory() get a relation() of Descendant (and sometimes even SubSelector). It should instead be
         // None.
         Relation relation() const { return static_cast<Relation>(m_relation); }
 
         bool isLastInSelectorList() const { return m_isLastInSelectorList; }
         void setLastInSelectorList() { m_isLastInSelectorList = true; }
         bool isLastInTagHistory() const { return m_isLastInTagHistory; }
         void setNotLastInTagHistory() { m_isLastInTagHistory = false; }
 
         // http://dev.w3.org/csswg/selectors4/#compound
         bool isCompound() const;
@@ skipping 210 matching lines @@
     if (m_hasRareData)
         return m_data.m_rareData->m_value;
     // AtomicString is really just a StringImpl* so the cast below is safe.
     // FIXME: Perhaps call sites could be changed to accept StringImpl?
     return *reinterpret_cast<const AtomicString*>(&m_data.m_value);
 }
 
 } // namespace WebCore
 
 #endif // CSSSelector_h