Appease mmtk

Author: byroot

gc.c

@@ -3087,7 +3087,10 @@ rb_gc_mark_children(void *objspace, VALUE obj)
     struct gc_mark_classext_foreach_arg foreach_args;
 
     if (rb_obj_exivar_p(obj)) {
-        gc_mark_internal(rb_obj_fields_no_ractor_check(obj));
+        VALUE fields_obj = rb_obj_fields_no_ractor_check(obj);
+        if (fields_obj) {
+            gc_mark_internal(fields_obj);
+        }
     }
 
     switch (BUILTIN_TYPE(obj)) {