infinite recursion in book_location_down() when there are multiple revisions

book_location_down() has a critical bug when book pages have multiple revisions. you can get into an infinite recursion if the book pages are moved around such that if at any time in the revision history, there's a circular parent/child relationship between any 2 pages. before zen comes in and moves this from "critical" to "exotic" ;) keep in mind i only found this bug because it happened on the live site at my day job. i wasn't trying to break anything, it just happened through the normal course of reorganizing book pages on our site. :(

luckily, the solution is trivial. we're doing an INNER JOIN on the book and node table, but have completely forgotten about the vid field (revision id). :( book stores records for each vid, so when we're SELECTing, we grab *all* revisions of all pages that have ever had the current node as its parent. instead, we only want to be looking at the most current versions of book pages that have the current nid as their parent. so, we just need to add an WHERE n.vid = b.vid clause to the INNER JOIN. patch attached (which applies cleanly to DRUPAL-4-7, and with a 5 line offset to HEAD).

please review/test and commit ASAP.

thanks!
-derek