[M52] add fail condition to addexpanded

(cherry-pick of 013e0e6d482f61181b829bf5ebfcad912c0061b1)

If coincident pairs don't match,
give up rather than deref null.

TBR=fmalita@chromium.org,caryclark@google.com
BUG=618991
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2058773002

Review-Url: https://codereview.chromium.org/2058773002
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2078183002