Enforce book node type and book[bid] checks

When looking for grant records of a book page node, a notice can show up if $node->book['bid'] is not set. This patch checks that node->type equals to 'book' in book_access_node_access_records(), enforces the isset($node->book['bid']), and fixes the signature.

The testcase I'm working on includes test for this issue. Probably all book page nodes include ->book['bid'], but creating a book page using node_save it may happen that book module does not create a complete ->book structure (possible core bug I'll post a follow up here), and this module shows a notice.