all stubs have valid major_keys, and this is OK. So I've added
a check to avoid termination in debug mode.

Review URL: http://codereview.chromium.org/572034

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3807 ce2b1a6d-e550-0410-aec6-3dcde31c8c00