Harden uniform array index parsing. Add overflow handling when parsing uniform variables. BUG=angleproject:2191 TEST=angle_unittests Change-Id: Ib2a69be1cc11a94420bc923a2aaaef8dc664d562 Reviewed-on: https://chromium-review.googlesource.com/756209 Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com> Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>