not yet initialized.

* variable.c (find_class_path): should initialize iv_tbl if it's
  NULL.

* class.c (rb_define_class): should return the existing class if
  the class is already defined and its superclass is ideintical to
  the specified superclass.

* class.c (rb_define_class_under): ditto.

* class.c (rb_define_module): should return the existing module if
  the module is already defined.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_6@1917 b2dd03c8-39d4-4d8f-98ff-823fe69b080e