[Perl/perl5] fce412: op.c: Add a cast to silence -Wsign-compare warning

[TAKAI Kousuke via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: fce4124fa70860bf2675923721139e9b376b0856
      
https://github.com/Perl/perl5/commit/fce4124fa70860bf2675923721139e9b376b0856
  Author: TAKAI Kousuke <62541129+t-...@users.noreply.github.com>
  Date:   2022-11-02 (Wed, 02 Nov 2022)

  Changed paths:
    M op.c

  Log Message:
  -----------
  op.c: Add a cast to silence -Wsign-compare warning

In 32-bit build (where IV is not wider than U32), comparing
PL_eval_begin_nest_depth and max_nest_iv would generate a warning:

    op.c:10817:42: warning: comparison of integer expressions of different 
signedness: ‘U32’ {aka ‘long unsigned int’} and ‘IV’ {aka ‘long int’} 
[-Wsign-compare]
    10817 |             if (PL_eval_begin_nest_depth >= max_nest_iv) {
          |                                          ^~