Cleanup: avoid deprecated behavior with volatile value

The warning can be seen here: https://godbolt.org/z/8M4qv7ezx

intern/guardedalloc/intern/mallocn_guarded_impl.cc

@@ -1341,7 +1341,8 @@ static const char *check_memlist(const MemHead *memh)
         membase->first = &backok->next;
       }
       else {
-        membase->first = membase->last = nullptr;
+        membase->first = nullptr;
+        membase->last = nullptr;
       }
     }
   }