-e: see title

This was raised by a user in https://github.com/Unity-Technologies/InputSystem/pull/1254.

Problem is that when you add a callback from a callback (e.g. add a new InputAction.started callback while the callbacks are being invoked), the newly added callback is immediately invoked.

Could be as simple as limited the number of invoked callbacks to at most the number we find when we start running through the list.