Fix subroutine unavailability during cloning

Fix subroutine unavailability during cloning

sub foo {
  my $x;
  format =
@
$x||'#'
.
}
write;
__END__
Variable "$x" is not available at - line 9.

That one’s OK.

sub foo {
  my sub x {};
  format =
@
&x
.
}
write;
__END__
Variable "&x" is not available at - line 9.
Assertion failed: (SvTYPE(_svmagic) >= SVt_PVMG), function S_mg_findext_flags, file mg.c, line 404.
Abort trap

That should say ‘Subroutine’.  And it shouldn’t crash.

The my-sub-cloning code was not taking this case into account.  The
value in the proto pad is an undef scalar.