If it's Smi::FromInt(0), the NULL check would trigger. Instead, use the
handle-zap value to mean "not set".

BUG=v8:3647,chromium:580651
R=vogelheim@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/1628173002

Cr-Commit-Position: refs/heads/master@{#33492}