Add bounds check to StringView::operator[] and StringView::charactersAt() as a hardening measure.
Pull request: https://github.com/WebKit/WebKit/pull/19386
Committed 269630@main (7eeecadfc008): <https://commits.webkit.org/269630@main> Reviewed commits have been landed. Closing PR #19386 and removing active labels.
<rdar://problem/117309123>