Fixed soundness bug reported by Sol Swords (thanks, Sol, for the example!) that was introduced after Version 6.4, when optimizing certify-book by reducing the "rolling back the logical world" as noted in :doc note-6-5.  Note: Have not yet run regression, but will do so immediately.

git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1274 5d84a466-a78b-0930-a311-59b22171a541