Fixes a bug in program ID validation for GetUniform* calls.

BUG=angle:571

Change-Id: I7080f0f7ea0def29d1b67c9d954ee5d9d41fb348
Reviewed-on: https://chromium-review.googlesource.com/228560
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>