Fix for: Fatal error: Call to a member function getParent() on a non-object in PropertyDescriptor.php

This method needs to be able to return false if $node is not an instance of Descriptor\PropertyDescriptor.

See also: https://github.com/phpDocumentor/phpDocumentor2/pull/1429
Merge pull request #1430 from gammamatrix/patch-3

Fix for: Fatal error: Call to a member function getParent() on a non-obj...