During scavanges multiple different ThinStrings can end up pointing to the
same internalized string. This means that the same key/value pair can be
re-inserted during rehash operations, so the values_[index] might not
be null when inserting the value.

BUG=chromium:695029

Change-Id: I5f693586bb60d5a2bbe89f05cb5b7f5fe26a23b2
Reviewed-on: https://chromium-review.googlesource.com/446380Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43379}