fix segfault in goops if class fields are redefined

fix segfault in goops if class fields are redefined

* libguile/goops.c (map, filter_cpl, remove_duplicate_slots): Use
  scm_is_pair instead of !scm_is_null, given that we use accessor
  macros.
  (check_cpl, build_slots_list): Check that descendents of <class> can't
  redefine slots of <class>.

* test-suite/tests/goops.test ("defining classes"): Add a test.

Patch originally by Stefan Israelsson Tampe.