* gc.c (gc_mark_maybe): added. check `is_pointer_to_heap()' and
  type is not T_ZOMBE.
* gc.c: use `gc_mark_maybe()'. T_ZOMBIE objects should not be pushed
  to the mark stack.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41345 b2dd03c8-39d4-4d8f-98ff-823fe69b080e