multi_buffer: Assert char boundary for panic due to point_to_buffer_offset (#40777)

In an attempt to figure out what's wrong with `point_to_buffer_offset`
for crash https://github.com/zed-industries/zed/issues/40453. We want to
know which branch among these two is the bad one.

Release Notes:

- N/A

Co-authored-by: Lukas Wirth <lukas@zed.dev>
10b9ae5e44Smit Barmase committed on 10/21/2025, 1:31:35 PM· committed by GitHubparentcad0601
3 files changedLine totals unavailable