disallow -4 pointer

A user's homegrown unsigned integer overflow tool
complains if a nullptr is decremented.
The conicWeight pointer likes to predecrement
before walking, but this is unnecessary if
its value is nullptr.

R=reed@google.com
BUG=skia:5415
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2061833005

Review-Url: https://codereview.chromium.org/2061833005
2 files changed