This fixes undefined behaviour with CFI.

Bug: chromium:1015810
Bug: angleproject:3162
Change-Id: I58cfb78adabbff05e5b4560dfd70b190411fa26d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1869303Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>