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.
Oh, I misread this guide.
http://book.cakephp.org/2.0/en/contributing/backwards-compatibility.html

I want to introduce this patch as a bug fix release (2.5.3 or 2.5.4)
And I think it should be backported into 1.3.x branch.

So, we shouldn't add a new public property basically.
I prefer $__safeUpdateMode to $safeUpdateMode.