[Perl/perl5] 60eec7: Add reference to av_extend_guts zeroing discussion

[Hugo van der Sanden via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 60eec70fe60786331e604b80d5a319b3b625a2b2
      
https://github.com/Perl/perl5/commit/60eec70fe60786331e604b80d5a319b3b625a2b2
  Author: Hugo van der Sanden <h...@crypt.org>
  Date:   2021-04-06 (Tue, 06 Apr 2021)

  Changed paths:
    M av.c

  Log Message:
  -----------
  Add reference to av_extend_guts zeroing discussion

.. via withdrawn PR 18690.