_place_monster_aux() was sometimes trying to place the monster out of bounds,

leading to an assertion.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5454 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on June 3, 2008
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