It's important that the test against the maximum of the valid range
is only done if the index is positive, so the sanitized index value
is guaranteed to end up in the valid range.

This fixes a regression from commit "Add GLSL support for
runtime-sized arrays in SSBOs".

BUG=chromium:789029
TEST=angle_unittests

Change-Id: Ic7125e383a64e46994b072df6d7e642432c521af
Reviewed-on: https://chromium-review.googlesource.com/792935Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>