Call mrb_full_gc before gc_each_objects; ref #3782

Otherwise dead object may be seen from `#each_object`.

src/gc.c

@@ -1483,6 +1483,7 @@ gc_each_objects(mrb_state *mrb, mrb_gc *gc, mrb_each_object_callback *callback,
 {
   mrb_heap_page* page;
 
+  mrb_full_gc(mrb);
   page = gc->heaps;
   while (page != NULL) {
     RVALUE *p;