Fix off-by-one error in Array.concat slow mode check

BUG=chromium:395499
LOG=n
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/399753005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22494 ce2b1a6d-e550-0410-aec6-3dcde31c8c00