Fix $DB::sub in DB::goto for lexical subs

$ PERL5DB='sub DB::DB{}' ./perl -Ilib -XMfeature=:all -de '
    state sub f{};
    sub DB::goto { warn $DB::sub }
    $^P|=0x80;
    sub {goto &f}->()'
main::f at -e line 1.

There should not be a package name.  Lexical subs should be treated
like anonymous subs here; $DB::sub should contain a reference.

This bug was introduced recently, in ae77754ae.  Before that the
output was:

CODE(0x7fdbf102de58) at -e line 1.

Though before 9d8e4b9b3 it was:

Segmentation fault: 11
perldelta for a5f477419
perldelta for 2872f9187
op.c: new is a keyword in C++