[Perl/perl5] e792c0: mg.c - fix $+ combined with branch reset

[Yves Orton via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: e792c028462d5d3771e15dff0293056e2eab3af6
      
https://github.com/Perl/perl5/commit/e792c028462d5d3771e15dff0293056e2eab3af6
  Author: Yves Orton <demer...@gmail.com>
  Date:   2023-03-08 (Wed, 08 Mar 2023)

  Changed paths:
    M mg.c
    M t/re/re_tests

  Log Message:
  -----------
  mg.c - fix $+ combined with branch reset

I missed that $+ needs to do the parno_to_logical lookup.
We had tests for $^N, but not $+. This also fixes the code
for $^N to only do the lookup when the paren is not 0.

Fixes #20912