bug#17355: #:getter procedure returns unexpected value in GOOPS

["Diogo F. S. Ramos" <dfsr@riseup.net>]

When using GOOPS, if a class has a second slot, the #:getter procedure
of the first slot returns the value of the second slot when applied to
an instance of a subclass.

--8<---------------cut here---------------start------------->8---
(use-modules (oop goops))

(define-class <foo> ()
  (a #:init-form 'foo #:getter foo-a)
  (b #:init-form 42))

(define-class <bar> (<foo>)
  (a #:init-form 'bar))
--8<---------------cut here---------------end--------------->8---

  (foo-a (make <foo>)) => foo
  (foo-a (make <bar>)) => 42

I expected:

  (foo-a (make <bar>)) => bar

I'm using Guile 2.0.11.