Fix GradientShaderBase4fContext::Interval fuzzer assert

Although the iterator ensures v0 != v1, 2 - v0 may still be equal to
2 - v1 for very small values due to limited float precision.

We need to recheck the inequality to avoid triggering Interval asserts.

BUG=skia:5903
R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2474463002

Review-Url: https://codereview.chromium.org/2474463002
1 file changed