Also fix one apparently-harmless bug in the Parser where we failed
to Finalize an empty scope. Without this fix, the DCHECK fails
on any C-style for-loop with a lexical binding that hits the
fast path (this is well-covered by many existing tests in mjsunit).

Thanks to Georg Neis for the suggestion of this DCHECK.

Change-Id: Ie1a8f8809f4d152c87f2da08209c610514645827
Reviewed-on: https://chromium-review.googlesource.com/587750Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47046}