Add validation for the pack buffer in ReadPixels BUG=angleproject:1512 Change-Id: Ia6bac628c35f04bc5d3adfde1569902475519698 Reviewed-on: https://chromium-review.googlesource.com/387668 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>