A surrogate pair split by an "always succeeding" backreference
(backreference capturing undefined, because it hasn't captured anything
yet) was incorrectly combined into a surrogate pair, resulting in
incorrect matches.
Bug: v8:13410
Change-Id: I2faf9ca5f9fcfd55cd6933a1ea038c88f8d3f524
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4013159
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#84276}
