Fix checking negative index when indexing matrix/vector 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/792935 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>