Rollup merge of #103521 - chenyukang:yukang/fix-103451-avoid-hang, r=jackh726,wesleywiser

Avoid possible infinite  loop when next_point reaching the end of file

Fixes #103451
If we return a span with `lo` = `hi`, `span_to_snippet` will always get `Ok("")`, which may introduce infinite loop if we don't care.

This PR make `find_width_of_character_at_span` return `width` with 1, so that `span_to_snippet` will get an `Err`.