ObjectSpace._id2ref should check liveness.

objspace->id_to_obj_tbl can contain died objects because of lazy
sweep, so that it should check liveness.

gc.c

@@ -3644,7 +3644,8 @@ id2ref(VALUE objid)
         }
     }
 
-    if (st_lookup(objspace->id_to_obj_tbl, objid, &orig)) {
+    if (st_lookup(objspace->id_to_obj_tbl, objid, &orig) &&
+        is_live_object(objspace, orig)) {
         return orig;
     }
 

test/ruby/test_objectspace.rb

@@ -35,6 +35,26 @@ End
   deftest_id2ref(false)
   deftest_id2ref(nil)
 
+  def test_id2ref_liveness
+    assert_normal_exit <<-EOS
+      ids = []
+      10.times{
+        1_000.times{
+          ids << 'hello'.object_id
+        }
+        objs = ids.map{|id|
+          begin
+            ObjectSpace._id2ref(id)
+          rescue RangeError
+            nil
+          end
+        }
+        GC.start
+        objs.each{|e| e.inspect}
+      }
+    EOS
+  end
+
   def test_count_objects
     h = {}
     ObjectSpace.count_objects(h)