Fix external snapshot reading by removing an assumption that strings in the snapshot file are null-terminated.

R=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/496253003

Patch from André Baixo <baixo@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23416 ce2b1a6d-e550-0410-aec6-3dcde31c8c00