class.c: check for malloc failures

* class.c (rb_class_subclass_add): use xmalloc
* class.c (rb_module_add_to_subclasses_list): ditto
* class.c (rb_class_remove_from_super_subclasses): use xfree
* class.c (rb_class_remove_from_module_subclasses): ditto
  [Bug #9616]

xmalloc does OOM checking and helps GC accounting when used
with xfree.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45302 b2dd03c8-39d4-4d8f-98ff-823fe69b080e