Don't stop when there are immediates left in NativeReactor

I can't seem to come up with a simple reproducer for this, but I managed to end up in a situation where there was only a single immediate scheduled, the reactor stopped itself because it did not appear in alarmOrder, and that immediate then scheduled a stream watcher, but the reactor was stopped so it was ignored.

I really wish I could make a reproducer for this, but I can't. All I know is that this fixes it.

 Sorry.