Only validate texture layer if texture is non-zero.
Only validate texture layer if texture is non-zero.

From the ES3.0 spec:
The error INVALID_VALUE is generated if texture is non-zero and layer
is negative.

BUG=765919

Change-Id: I0ac9bd0335ab4d55701d6def0158297d5add993e
Reviewed-on: https://chromium-review.googlesource.com/723741
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Yuly Novikov <36f5580f63d0a3eacc17e89a61d11e2f646630d2@chromium.org>
Commit-Queue: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle