Problem: charidx() and utf16idx() result is not consistent with byteidx().
Solution: When the index is equal to the length of the text return the
lenght of the text instead of -1. (Yegappan Lakshmanan,
closesvim/vim#12503)
577922b917
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>