Add additional protection against OOB accesses by masking the
index to access by a mask precomputed from the memory size.

R=clemensh@chromium.org, bradnelson@chromium.org

Change-Id: I1d5875121e1904074b115a2c88ca773b6c1c1a66
Reviewed-on: https://chromium-review.googlesource.com/830394Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50162}