[Perl/perl5] 2f1617: locale.c: Don't use SvPVX without checking

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 2f1617ef716ae12369475fa6c11cbefd7ba7dfca
      
https://github.com/Perl/perl5/commit/2f1617ef716ae12369475fa6c11cbefd7ba7dfca
  Author: Karl Williamson <k...@cpan.org>
  Date:   2023-10-20 (Fri, 20 Oct 2023)

  Changed paths:
    M locale.c

  Log Message:
  -----------
  locale.c: Don't use SvPVX without checking

It turns out that compile with -Accflags=-DNO_LOCALE_CONV can cause
localeconv() to return SVs that are of SVt_NULL type, so doing an SvPVX
can core dump.  SvPV_nolen will coerce the non-string into an empty one
so that we don't have to care.