Don't use the random_uselessness() miscast effect during arena mode.

If a melee miscast kills the attacker don't let it get any more hits in.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8261 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on January 5, 2009
WPCL2ABFWUA7JQUJSGS267ZV3V6F6Y24DRLD23Q3LT3NDSWYH6JAC
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