This undefined behavior appeared in angle_end2end_tests compiled with
clang 3.6 on Mac.

BUG=angleproject:891

Change-Id: I840b40a7bd54c9cfaeb0fe6784b73f875f9d5502
Reviewed-on: https://chromium-review.googlesource.com/303337Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>