Check if the parent actually loaded, before trying to set its thumbnail...

In batches, it's easier to first ingest all the pages, before the book object...
...  because the book object didn't exist, it was causing the batch to break.
Update islandora_book.module

Avoid reusing a variable.
Merge pull request #57 from adam-vessey/7.x

Check if the parent actually loaded, before trying to set its thumbnail....