This error was not exposed since compressed formats use different validation
paths.  Fixed for completeness.

BUG=angleproject:1511

Change-Id: I9b44a795b1e43f52555440f776e00bb4b92354ef
Reviewed-on: https://chromium-review.googlesource.com/382653
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>