Fixes edge cases for parsing / serializing codepoint 0xffff in JSON
with UTF16.

New Rev: 0213a8545f6362cd1cd5091cedf29747736552e8

Change-Id: I48b174cf1bd9263ace002996094f7143a1248766
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2020502
Auto-Submit: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65985}