Don't dereference null pointer in FastTextAutosizer::getTentativeClusterRoots()

Source/core/rendering/FastTextAutosizer.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 195 matching lines @@
 
     // Fingerprints are computed during style recalc, for (some subset of)
     // blocks that will become cluster roots.
     class FingerprintMapper {
     public:
         void add(const RenderObject*, Fingerprint);
         void addTentativeClusterRoot(const RenderBlock*, Fingerprint);
         // Returns true if any BlockSet was modified or freed by the removal.
         bool remove(const RenderObject*);
         Fingerprint get(const RenderObject*);
-        BlockSet& getTentativeClusterRoots(Fingerprint);
+        BlockSet* getTentativeClusterRoots(Fingerprint);
     private:
         typedef HashMap<const RenderObject*, Fingerprint> FingerprintMap;
         typedef HashMap<Fingerprint, OwnPtr<BlockSet> > ReverseFingerprintMap;
 
         FingerprintMap m_fingerprints;
         ReverseFingerprintMap m_blocksForFingerprint;
 #if ENABLE(ASSERT)
         void assertMapsAreConsistent();
 #endif
     };
@@ skipping 74 matching lines @@
     FingerprintMapper m_fingerprintMapper;
     Vector<RefPtr<RenderStyle> > m_stylesRetainedDuringLayout;
     // FIXME: All frames should share the same m_pageInfo instance.
     PageInfo m_pageInfo;
     bool m_updatePageInfoDeferred;
 };
 
 } // namespace blink
 
 #endif // FastTextAutosizer_h