Correct err msg when calling stub w/no autoload fb

Correct err msg when calling stub w/no autoload fb

If an AUTOLOAD subroutine loads a sub by assigning to the glob, there
may be code elsewhere that has a reference to a stub, that is now
assigned over.  To cope with this situation, calls to undefined sub-
routines will fall back to whatever sub is in the subroutine’s owner
typeglob.  This has been the case since Perl 5.000.

But the error message that occurs if the typeglob happens to have no
sub in it is wrong:

$ perl -e '
   my $foosub = \&foo;
   undef *foo;
   &$foosub;
  '
Not a CODE reference at -e line 4.

as opposed to this:

$ perl -e '
   my $foosub = \&foo;
   &$foosub;
  '
Undefined subroutine &main::foo called at -e line 3.

They should both produce the same error message, because $foosub is a
code reference, albeit without a body.