Fix bug #2499021: during arena mode, make monster_random_space() only pick

spaces inside the arena viewport. Since rock worms can be teleported into non-permanent rock walls, and the entire level outside the arena is filled with non-permanent rock walls, this prevents rock worms from being teleported outside the arena.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8419 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on January 11, 2009
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