Skip nullptr after copyToVector() from HeapHashSet<WeakMember>

This patch checks and skips nullptr after copyToVector() from
HeapHashSet<WeakMember>.

This is a speculative fix.

BUG=581698, 689949, 696231

Review-Url: https://codereview.chromium.org/2723033002
Cr-Commit-Position: refs/heads/master@{#453890}
2 files changed