Ensure that unmap return values are checked in debug mode.

R=erik.corry@gmail.com
BUG=v8:1701

Review URL: http://codereview.chromium.org/8060052

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9480 ce2b1a6d-e550-0410-aec6-3dcde31c8c00