Fix Heap-use-after-free READ 2 in Intl.Segmenter
when the segments got free during the iteration
We need to keep a copy of the string in the iterator instead
of depending on the one referenced from the segments.

Bug: chromium:1121156, v8:6891
Change-Id: I26ef5baccaa470dc1bd8cc229c737f556d27160e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2376173
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69575}