[Perl/perl5] 7b8a05: Set errno when my_strftime() fails.

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 7b8a05c44312f85b9aaac73df47258384daf2a8a
      
https://github.com/Perl/perl5/commit/7b8a05c44312f85b9aaac73df47258384daf2a8a
  Author: Karl Williamson <k...@cpan.org>
  Date:   2023-08-08 (Tue, 08 Aug 2023)

  Changed paths:
    M locale.c

  Log Message:
  -----------
  Set errno when my_strftime() fails.

The libc strftime() doesn't have any way to distinguish between
erroneous input and that which just doesn't generate any output; hence
it doesn't set errno.

But my_strftime() has logic to reasonably confidently make that
distinction, so it should set errno when it determines that it is
erroneous input.