Fix [2777074]: Properly add the negative dexterity effects from certain

scale mutations instead of subtracting them.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9682 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on April 22, 2009
BP6JDWI7X4IOEKUM65X33IPBCQOY27JEY4SQ2LZ7QQUPMT2DH66AC
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