[Perl/perl5] 19f71f: Do not cache utf8 offsets for non-canonical lengths

[Leon Timmermans via perl5-changes]

  Branch: refs/heads/leont/utf8-index
  Home:   https://github.com/Perl/perl5
  Commit: 19f71fa8688fc8f44d3457746f5f8d0b0277a61e
      
https://github.com/Perl/perl5/commit/19f71fa8688fc8f44d3457746f5f8d0b0277a61e
  Author: Leon Timmermans <faw...@gmail.com>
  Date:   2021-04-19 (Mon, 19 Apr 2021)

  Changed paths:
    M embed.fnc
    M proto.h
    M sv.c
    M t/op/index.t

  Log Message:
  -----------
  Do not cache utf8 offsets for non-canonical lengths

In particular, if the length is beyond the end, it should not be stored as the 
end.