On 32-bit systems, the computation {count + type_list->size()} can
overflow, leading to memory corruption later on.

R=titzer@chromium.org

Bug: chromium:819869
Change-Id: Ic81d201e58211e3989b4e945cd52e98dc951fbda
Reviewed-on: https://chromium-review.googlesource.com/955025
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51817}