perly LSTOPSUB: prevent a double op free between parse stack and CV

The reeproducer resulted in the "block" OP being both on the parser
stack and attacked to the CV.  If an error occurred while parsing the
rest of the list operator clean up would release the OP as attached
to the CV, and the same OP on the parse stack, resulting in a double
free.

It's unclear to me whether bison is intended to support modifying
the parse stack entry like this, but it appears to work here.

Fixes Perl#21724

MANIFEST

@@ -6066,6 +6066,7 @@ t/lib/croak/class			Test croak calls from class.c
 t/lib/croak/gv				Test croak calls from gv.c
 t/lib/croak/mg				Test croak calls from mg.c
 t/lib/croak/op				Test croak calls from op.c
+t/lib/croak/parser			Test parser errors
 t/lib/croak/pp				Test croak calls from pp.c
 t/lib/croak/pp_ctl			Test croak calls from pp_ctl.c
 t/lib/croak/pp_hot			Test croak calls from pp_hot.c