Prevent overflow in the variable packer by sanity checking each variable. BUG=346489 Change-Id: I28f5751580729a4d4d77fa6fdee0b4a6628a05de Reviewed-on: https://chromium-review.googlesource.com/188030 Tested-by: Nicolas Capens <nicolascapens@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org>