[Perl/perl5] a22b3a: locale.c: Add missing check for error return

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: a22b3a763ec32843d36c7900bf0687979cc57d33
      
https://github.com/Perl/perl5/commit/a22b3a763ec32843d36c7900bf0687979cc57d33
  Author: Karl Williamson <k...@cpan.org>
  Date:   2024-01-29 (Mon, 29 Jan 2024)

  Changed paths:
    M locale.c

  Log Message:
  -----------
  locale.c: Add missing check for error return

When strftime() fails, it returns NULL; the check for this was missing,
which led to a segfault on Windows that didn't appear on other platforms
because the particular test worked on other platforms.

Fixes GH #21880