[Perl/perl5] 12865d: mg.c: Fix to compile under g++

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 12865d70b33295cc1e31678404f5aef517d3a97b
      
https://github.com/Perl/perl5/commit/12865d70b33295cc1e31678404f5aef517d3a97b
  Author: Karl Williamson <k...@cpan.org>
  Date:   2022-06-11 (Sat, 11 Jun 2022)

  Changed paths:
    M mg.c

  Log Message:
  -----------
  mg.c: Fix to compile under g++

Commit 00a5df846e035280750985222a693ac58022ee36 fixed some c undefined
behavior, but it will not compile with g++, because a goto crosses
initialization.

https://stackoverflow.com/questions/14274225/statement-goto-can-not-cross-variable-definition

and its links sort of explains why

This commit works around this limitation