fixed overflow detection in StringValue::operator[]

StringValue.cpp

@@ -25,7 +25,7 @@ void StringValue::operator--()
 
 const char& StringValue::operator[](size_t index) const
 {
-    if (index > _size-1)
+    if (index > _size - 2)
         throw std::out_of_range("Index out of range!");
     return _data[index];
 }