[Perl/perl5] 973873: Eliminate build-time warning

[James E Keenan]

  Branch: refs/heads/smoke-me/jkeenan/zefram/ghi-17664-opcode
  Home:   https://github.com/Perl/perl5
  Commit: 9738738e1fdf7750d02a04365aa5c09f1792e90d
      
https://github.com/Perl/perl5/commit/9738738e1fdf7750d02a04365aa5c09f1792e90d
  Author: Zefram <zef...@fysh.org>
  Date:   2020-03-27 (Fri, 27 Mar 2020)

  Changed paths:
    M ext/Opcode/Opcode.pm
    M ext/Opcode/Opcode.xs

  Log Message:
  -----------
  Eliminate build-time warning

Compiling with gcc7, for example, generated a '-Woverflow' warning with
text 'overflow in implicit constant conversion'.

For: https://github.com/Perl/perl5/issues/17664