[perl #71154] undef &$coderef consistency

[perl #71154] undef &$coderef consistency

$ perl -le' undef &{$x=sub{}}; $x->()'
Not a CODE reference at -e line 1.

undeffing an anonymous subroutine used to turn the ANON flag off,
causing it to bypass the condition apparently written for this situa-
tion in pp_entersub.

This commit stops cv_undef from turning off that flag, so now we get:

$ ./perl -le' undef &{$x=sub{}}; $x->()'
Undefined subroutine called at -e line 1.