Don't use Vector<UChar> to build strings

Source/wtf/text/StringBuffer.h

Index: Source/wtf/text/StringBuffer.h
diff --git a/Source/wtf/text/StringBuffer.h b/Source/wtf/text/StringBuffer.h
index 9af42dc23666528f2faa999d81d0ea723af218f5..74ad12f6070d19bc9fd5661c568c08894702cc36 100644
--- a/Source/wtf/text/StringBuffer.h
+++ b/Source/wtf/text/StringBuffer.h
@@ -70,7 +70,7 @@ public:
     }
 
     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]; }