The array size for unsized arrays needs to be sanitized in all cases,
since subsequent checks on array indexing depend on all arrays being
sized.

BUG=chromium:781729
TEST=angle_unittests

Change-Id: I3af6c08bb249a19f7c2ef169c877a2b863eb31d3
Reviewed-on: https://chromium-review.googlesource.com/757101
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>