[Perl/perl5] 08be3e: skip trying to constant fold an incomplete op tree

[Tony Cook via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 08be3ef7f1190d94279ad0b3e13519ac8dc3b0ec
      
https://github.com/Perl/perl5/commit/08be3ef7f1190d94279ad0b3e13519ac8dc3b0ec
  Author: Tony Cook <t...@develop-help.com>
  Date:   2020-12-23 (Wed, 23 Dec 2020)

  Changed paths:
    M op.c
    M t/op/cmpchain.t

  Log Message:
  -----------
  skip trying to constant fold an incomplete op tree

This code would try to constant fold an op tree like

  relop
    +- null
    +- constant

which would underflow the stack, potentially crashing perl.

This is intended as a quick fix rather than as a complete
solution.

Fixes #18380