Fixes signed/unsigned mismatch due to missing type cast inside an
assert.

BUG=angleproject:1382
TEST=standalone MSVS 2015 debug build

Change-Id: I1508b54a2608e9204ad582bde4bd62af6926c36e
Reviewed-on: https://chromium-review.googlesource.com/360921Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>