gc.c: zombie is not alive

* gc.c (is_live_object): finalizer may not run because of lazy-sweep.
  [ruby-dev:47786] [Bug #9069]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43502 b2dd03c8-39d4-4d8f-98ff-823fe69b080e