[Perl/perl5] 08e55e: win32: fix waitpid(-1, WNOHANG) segfault/panic

[xenu]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 08e55ec5e3ef6d6c040c0dc8bdec7d59f76bfbe8
      
https://github.com/Perl/perl5/commit/08e55ec5e3ef6d6c040c0dc8bdec7d59f76bfbe8
  Author: Tomasz Konojacki <m...@xenu.pl>
  Date:   2019-11-10 (Sun, 10 Nov 2019)

  Changed paths:
    M win32/win32.c

  Log Message:
  -----------
  win32: fix waitpid(-1, WNOHANG) segfault/panic

waitpid(-1, WNOHANG) would panic or segfault if called when the
thread's message queue is not empty.

Thanks to Erik Jezierski for the report and diagnosis.

[gh #16529]