mrb_class_real() did not work for BasicObject; fix #4037

src/class.c

@@ -1763,10 +1763,10 @@ mrb_class_path(mrb_state *mrb, struct RClass *c)
 MRB_API struct RClass*
 mrb_class_real(struct RClass* cl)
 {
-  if (cl == 0)
-    return NULL;
+  if (cl == 0) return NULL;
   while ((cl->tt == MRB_TT_SCLASS) || (cl->tt == MRB_TT_ICLASS)) {
     cl = cl->super;
+    if (cl == 0) return NULL;
   }
   return cl;
 }