We were missing a check for an out-of-bounds Uniform, which could
cause us to miss an error and also give assertion failures.

BUG=angle:571

Change-Id: I844658a82ce13ea179a2358a202733c54bc2c0c7
Reviewed-on: https://chromium-review.googlesource.com/212934Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>