DoStoreKeyedFastElement was passing incorrect slot address (off by 1) to the RecordWrite.

R=erik.corry@gmail.com
BUG=v8:1742

Review URL: http://codereview.chromium.org/8139026

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9528 ce2b1a6d-e550-0410-aec6-3dcde31c8c00