Fix SkTwoPointConicalGradient zero-radius handling

r == 0 is within valid gradient range, we shouldn't skip it.

BUG=skia:5023
R=caryclark@google.com,reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1756573002

Review URL: https://codereview.chromium.org/1756573002
2 files changed