We should never allocate a 0-sized buffer, so never grow from 0.

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/446933003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22947 ce2b1a6d-e550-0410-aec6-3dcde31c8c00