Renamed all variables named 'null' (for real now, and only that)

src/scanner.cpp

@@ -383,7 +383,7 @@ namespace YAML
 	// . Does not parse any more tokens.
 	void Scanner::ThrowParserException(const std::string& msg) const
 	{
-		Mark mark = Mark::null();
+		Mark mark = Mark::null_mark();
 		if(!m_tokens.empty()) {
 			const Token& token = m_tokens.front();
 			mark = token.mark;