Following dolorous's comments on 2488905, use random_in_bounds() instead

of manual randomization. Also fixes the bug where this was biased towards the bottom-right section of the map. Unfortunately, it doesn't fix 2488905.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8750 c06c8d41-db1a-0410-9941-cceddc491573

Created by  haranp  on January 25, 2009
KMID7RZF737CIGE4TTZPJIXVOL5JSXG6AWXVKGLNPXARNPHJEBCAC
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