Check that object is live in each_object

Fixes crash in:

    10_000.times.map do
      Object.new
    end

    run_gc = true

    ObjectSpace.each_object do |o|
      if run_gc
        GC.start
        run_gc = false
      end
      puts o
    end

gc/mmtk.c

@@ -848,6 +848,8 @@ each_object(struct objspace *objspace, int (*func)(VALUE, void *), void *data)
 
     VALUE *obj_ptr;
     rb_darray_foreach(objs, i, obj_ptr) {
+        if (!mmtk_is_live_object((MMTk_ObjectReference)*obj_ptr)) continue;
+
         if (func(*obj_ptr, data) != 0) {
             break;
         }