Fix silly monster AI bug introduced in r9129. Oops. Fixes [2635574].

Also fix mons_near() to be more correct.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9213 c06c8d41-db1a-0410-9941-cceddc491573

Created by  haranp  on February 25, 2009
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