Fix uninitialized FixedArray potentially being left behind by ElementsTransitionGenerator::GenerateDoubleToObject

BUG=chromium:421843
LOG=n
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/636313003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24498 ce2b1a6d-e550-0410-aec6-3dcde31c8c00