And apply a minor change: the {end_offset} is currently always >= 1, and
we sometimes use {end_offset - 1}. Change this to compute the
{end_offset} to be one less than before, and use {Uint32LessThan}
instead of {Uint32LessThanOrEqual}.
This matches the documentation I added and makes reasoning about the
correctness of the checks easier (at least for me).

R=titzer@chromium.org

Change-Id: I9a18ad5c72895cbadb6593cb74d6edc24f9ab032
Reviewed-on: https://chromium-review.googlesource.com/852145
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50411}