This is a quick-fix for an ICE in `tabs_in_doc_comments`. The problem
was that we we're indexing into possibly multi-byte characters, such as '位'.
More specifically `get_chunks_of_tabs` was returning indices into
multi-byte characters. Those were passed on to a `Span` creation that
then caused the ICE.
This fix makes sure that we don't return indices that point inside a
multi-byte character. *However*, we are still iterating over unicode
codepoints, not grapheme clusters. So a seemingly single character like y̆ ,
which actually consists of two codepoints, will probably still cause
incorrect spans in the output.
