fix a Zeroing Weak References problem

When I use PXSourceList In `osx 10.7`,  my program will crash exactly like [this problem](http://stackoverflow.com/questions/10904990/xmpp-objective-c-library-cannot-form-weak-reference-to-instance-0x7fcd4a49893) .

I make [a demo](https://github.com/CrazyCatcher/PXErrorDemo) for this problem.

Please see if that is this the right way to  solved the problem.