Avoid undefined behavior and enable asserts in ClampTest.

Avoid undefined behavior and enable asserts in ClampTest.

The old code here wasn't being careful to avoid int32_t overflow in slow_check.  Fix that.

R_ASSERT hasn't been doing anything for a while.  As a result, there are a couple bugs in SkClampRange, marked as such and commented out.  The asserts also weren't quite passing, so I fixed them up (allowing 0xFFFF to be considered either as part of the ramp or part of V1.)

BUG=skia:2481
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/260523004

git-svn-id: http://skia.googlecode.com/svn/trunk@14479 2bbb7eff-a529-9590-31e7-b0007b416f81