Don't use Vector<UChar> to build strings

Source/wtf/text/StringBuffer.h

 /*
  * Copyright (C) 2008, 2010 Apple 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer. 
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 52 matching lines @@
         if (newLength > m_data->length()) {
             RELEASE_ASSERT(newLength <= std::numeric_limits<unsigned>::max() / sizeof(UChar));
             CharType* characters;
             m_data = StringImpl::reallocate(m_data.release(), newLength, characters);
             return;
         }
         shrink(newLength);
     }
 
     unsigned length() const { return m_data->length(); }
-    CharType* characters() { return const_cast<CharType*>(m_data->getCharacters<CharType>()); }
+    CharType* characters() { return length() ? const_cast<CharType*>(m_data->getCharacters<CharType>()) : 0; }
 
     CharType& operator[](unsigned i) { ASSERT_WITH_SECURITY_IMPLICATION(i < length()); return characters()[i]; }
 
     PassRefPtr<StringImpl> release() { return m_data.release(); }
 
 private:
     RefPtr<StringImpl> m_data;
 };
 
 } // namespace WTF
 
 using WTF::StringBuffer;
 
 #endif // StringBuffer_h