message: Fix masked reduce vcall for single self pointer
diff: @@ -56,8 +56,13 @@ Result vcall_jit_reduce_impl(Func func, const Self &self_,
  schedule(args...);
  size_t self_size = self_.size();
  - if (self_size == 1)
  - return func(self_.entry(0), args...);
  + if (self_size == 1) {
  + auto self = self_.entry(0);
  + if (self)
  + return func(self, args...);
  + else
  + return zero<Result>();
  + }
  Mask mask = extract_mask<Mask>(args...);
  if (jit_var_mask_size(Backend))
  