Fix GetUniform validation. 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/212934 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>