Add null check for format in TIXML_VSCPRINTF (#1064)

Author: sbaluja

tinyxml2.cpp

@@ -114,6 +114,9 @@ distribution.
 	#define TIXML_VSNPRINTF	vsnprintf
 	static inline int TIXML_VSCPRINTF( const char* format, va_list va )
 	{
+	    if (!format) {
+	        return 0;
+	    }
 		int len = vsnprintf( 0, 0, format, va );
 		TIXMLASSERT( len >= 0 );
 		return len;