[Perl/perl5] 85bf3b: Change loop type to avoid compiler warnings

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 85bf3b944a4989111c23c43f6ee53a3c198e6412
      
https://github.com/Perl/perl5/commit/85bf3b944a4989111c23c43f6ee53a3c198e6412
  Author: Karl Williamson <k...@cpan.org>
  Date:   2022-07-02 (Sat, 02 Jul 2022)

  Changed paths:
    M regexec.c

  Log Message:
  -----------
  Change loop type to avoid compiler warnings

On some configurations we're getting comparison of signed vs unsigned.

This fixes GH #19915