Protect book pages - patch attached

I installed this module on an Open Atrium site and immediately felt a sense of relief. Thanks for writing it.

One issue I had was that if I changed the audience of a sub-page in a book (the Notebook feature in OA) then that sub-page became unreachable. The same happened to sub-pages if I changed the audience of their parent.

I don't know whether this behaviour is specific to my setup but the quickest way to stop it happening seemed to be to hack the module. I've attached my patch in case it's useful to anyone else.