Disallow randart slings of venom, since they can produce poisoned stones

or sling bullets.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5013 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on May 12, 2008
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