res_poison() returns -1 for vulnerability, which evaluates to true, so

check for res_poison() > 0 instead.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8462 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on January 15, 2009
MQR3TGO47US7ZGIRLLDZMB6C32ATY6K2ZR5LGXLOFTFRALSYYZWQC
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