aio: do not reschedule or start operations when expire queue is exiting

We do not want to let operations restart if we're in the process of shutting
down.  This ensures that they get a reasonable hard failure.

src/core/aio.c

@@ -343,7 +343,7 @@ nni_aio_begin(nni_aio *aio)
 	aio->a_cancel_fn = NULL;
 
 	// We should not reschedule anything at this point.
-	if (aio->a_stop) {
+	if (aio->a_stop || eq->eq_exit) {
 		aio->a_result    = NNG_ECANCELED;
 		aio->a_cancel_fn = NULL;
 		aio->a_expire    = NNI_TIME_NEVER;
@@ -380,7 +380,7 @@ nni_aio_schedule(nni_aio *aio, nni_aio_cancel_fn cancel, void *data)
 	}
 
 	nni_mtx_lock(&eq->eq_mtx);
-	if (aio->a_stop) {
+	if (aio->a_stop || eq->eq_exit) {
 		nni_task_abort(&aio->a_task);
 		nni_mtx_unlock(&eq->eq_mtx);
 		return (NNG_ECLOSED);