page shouldn't have itself as parent

1. Create a page named 'mypage' and save it.
2. Modify 'mypage' and change ID for 'foobar' and set parent as 'mypage'

You now have a page having itself as parent.

Try to edit or to view that page. You get an infinite loop, then a memory error.

There should be a mechanism to prevent that, both in forms and in page loading mechanism.