A buffer size validation check was returning an invalid
operation error based on buffer size, even when primcount
is <= 0 (so the buffer isn't used).

Fixes the following WebGL 1.0.4 test with SwANGLE:
conformance/extensions/angle-instanced-arrays-out-of-bounds.html

Bug: b/154628007
Change-Id: Ibfe4eb57f17cd07100fb8dfbe8069e9ec7515160
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2168546Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>