[aot] Runtime device is null by surprise with too many TiArgument s

[PENGUINLIONG]

This is exposed by the following usage:

ti::Kernel kernel = ...
for (int i = 0; i < N_ITER; ++i) {
  // user forgot to `kernel.clear_args();` here.
  kernel.push_arg(i);
  kernel.launch();
}

cc @ailzhang