The prior check was incorrectly flagging index=0/count=2 as
invalid. It also missed a few cases like a shard count of 0.

Bug: angleproject:3162
Change-Id: Ida4c0f454ece15e4304d024d205475acf0f135e6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2423207Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>