In cv_clone, use pad ID to identify mysub outside

In cv_clone, use pad ID to identify mysub outside

This code prints ARRAY(0x802e10), whereas it should print
SCALAR(0xfedbee):

undef &bar;
eval 'sub bar { my @x }';
{
  my sub foo;
  foo();
  sub bar {
    CORE::state $x;
    sub foo { warn \$x }
  }
}

The foo sub has a strong CvOUTSIDE pointer, but what it points to
can still be undefined and redefined.  So we need to identify it
by its pad.