Metal: Clamp vector element index references Ensured that vector elements passed by reference are within the number of vector components. Fixed: angleproject:8492 Change-Id: I2defbd8dfd418256ddb87f8ce1a74ac09a8a1b7b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5229957 Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>