Add function can_safely_mutate(), to determine whether a player has any

chance of rotting upon mutation, and make Xom's random mutation routines use it.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@4482 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on April 22, 2008
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