stash_filter shouldn't affect artefacts.
Don't try to cache the names for BOOK_RANDART_LEVEL and BOOK_RANDART_THEME, since randarts don't have a single name.
git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7990 c06c8d41-db1a-0410-9941-cceddc491573
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