Add overflow handling when parsing uniform variables.

BUG=angleproject:2191
TEST=angle_unittests

Change-Id: Ib2a69be1cc11a94420bc923a2aaaef8dc664d562
Reviewed-on: https://chromium-review.googlesource.com/756209Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>