[M54] Improve ColorStopOptimizer safety

Could potentially access uninitialized memory.

M54 cherry-pick for http://crbug.com/647481

BUG=647481
TBR=brianosman@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2345343002

Review-Url: https://codereview.chromium.org/2345343002
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2405853002
1 file changed