Similar to the checks added in https://bugs.webkit.org/show_bug.cgi?id=149982, check if start and end positions are still valid after updating them through mergeEndWithNextIfIdentical.
Created attachment 442201 [details] Patch
Committed r284739 (243448@main): <https://commits.webkit.org/243448@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 442201 [details].
<rdar://problem/84574168>