libaurum: make an uiobject not to use an invalid inner handler.

libaurum: make an uiobject not to use an invalid inner handler.

An atspi object can be removed even if ref count isn't zero, when a parent window is destroyed.
on object destroyed callback, uiobject set invalid flag by itself
to prevent crahsing by calling atspi api with invalid handler.

Change-Id: Ifb8efee7b97d46e418404aea30198a72ebe90fcd