Change assert to release assert for WTF::double_conversion::Vector to prevent OOB memory access.

      R=inferno@chromium.org, mbarbella@chromium.org, ochang@chromium.org, tkent@chromium.org
BUG=574802

Review URL: https://codereview.chromium.org/1677363002

Cr-Commit-Position: refs/heads/master@{#374424}