skip zero-length linear gradient intervals to make new code more consistent with older table-based implementation

BUG=skia:4841
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1619413002

Review URL: https://codereview.chromium.org/1619413002
1 file changed