Fix off-by-one error in StairConnectivity access (player_branch_depth() is 1-based).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7478 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on November 18, 2008
55WY6PPY53OIQGF7O772YFCHS2EW76NHJ3TUPND53MXFBPJJON4AC
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