COObject: Add an assertion that will fail if you attempt to access the variable storage of a 'zombie' COObject instance. Adding this now because I had a bug involving 'zombie' objects (not loaded in an object graph context) and it was very confusing to debug

git-svn-id: svn://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@9972 7b3f36aa-e6db-49c1-8b4c-66eac1997e64