Check string range in `ShapeSegment`

crrev.com/c/5776342 fixed a range `CHECK` in
`CollectFallbackHintChars`, but depends on the CSS and font
configurations, it's possible that the code doesn't go to
`CollectFallbackHintChars` and the following code may hit
the same issue.

This patch adds another `CHECK` for the case.

Bug: 355731798, 357622693
Change-Id: Ieb4ada7699c80564e8a4b866cb6a6ffbc665ebc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5776204


Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1340006}