[CXMLNode dealloc] EXC_BAD_ACCESS, revisited

As noted in the last few posts in [https://github.com/TouchCode/TouchXML/issues/11](https://github.com/TouchCode/TouchXML/issues/11), the EXC_BAD_ACCESS error still occurs intermittently in the iOS simulator. This suggested fix (loop to invalidate CXMLDocument node pool upon dealloc) appears to work.