Fix out of range access in MemoryBarrierTest. Bug: angleproject:6646 Change-Id: Ia406e290d0a6d6bef6352ac5e46a591e980d33f3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3258006 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>