avoid a couple divide by zeroes so cheaters don't crash when "winning"

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@4193 c06c8d41-db1a-0410-9941-cceddc491573

Created by  pauldubois  on April 11, 2008
BEN5BFGZMJFKU35GFCQ34EPBA7Z3NPZCUHIE3SZHU55CA4G4DQPAC
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