Fix cluster-fuzz found regression with d8 Workers

Fix cluster-fuzz found regression with d8 Workers

This one occurred when serializing an object. When the property getter threw an
exception, that value was skipped, but the property count wasn't updated. The
deserializer then tried to deserialize the wrong value.

BUG=chromium:506549
R=jarin@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1220193004

Cr-Commit-Position: refs/heads/master@{#29541}