Add bounds check to StringImpl::operator[] for extra safety.
Pull request: https://github.com/WebKit/WebKit/pull/19723
Committed 269967@main (e93f8aa0c4f2): <https://commits.webkit.org/269967@main> Reviewed commits have been landed. Closing PR #19723 and removing active labels.
<rdar://problem/117700025>
*** Bug 263574 has been marked as a duplicate of this bug. ***