gc.c: mark live objects only

* gc.c (wmap_mark_map): mark live objects only, but delete zombies.
  [ruby-dev:47787] [Bug #9069]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43509 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
fix typo

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43510 b2dd03c8-39d4-4d8f-98ff-823fe69b080e