Fix rare access violation during JS heap serialization.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/510013002

Patch from Slava Chigrin <vchigrin@yandex-team.ru>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23488 ce2b1a6d-e550-0410-aec6-3dcde31c8c00