Fixed: If can’t add child, don’t add to child info

Fixed: If can’t add child, don’t add to child info

If the call to set_parent() failed, we were still adding the child to
the internal list of children, despite that it was not really added.
That meant we could later try to do invalid stuff with that non-child.

Fix that by asserting and giving up if the child that the user is
attempting to add is already parented.

https://bugzilla.gnome.org/show_bug.cgi?id=701296