Fix a race condition problem

I've fixed #3857.

And I am sorry. I didn't know my commit was notified to upstream automatically. 
I shouldn't have done force-push.
I worry about unintended overriding.
And this property is used only internally. In that sense, It is a private property. But friend classes (like a C++)  are allowed to access this property.

But reading this guide, could I add a public property without any concern?
http://book.cakephp.org/2.0/en/contributing/backwards-compatibility.html#working-on-cakephp