Incorrect ordering lead to crashes during GC when end position was not even number.

Add static assert to catch this kind of typos.

Review URL: http://codereview.chromium.org/4640003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5776 ce2b1a6d-e550-0410-aec6-3dcde31c8c00