We really just need one check instead of three. This also unifies the
error message to be the same on 32 and 64 bit systems.

Drive-by: Fix potential overflow in {validate_size}.

R=titzer@chromium.org

Bug: chromium:794353
Change-Id: I63c1f5ef53c1f245b9e82bcbf86a5d9ac0d2725e
Reviewed-on: https://chromium-review.googlesource.com/824082Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50071}