aio: run callback from aio_begin if stopped

There is a very subtle case where an aio can be closed, but we
don't realize it until precisely the moment of nni_aio_begin.
This is eaten, and the submitter winds up having no way of knowing
that the aio is closed, leading to a potential hang, or a leaked
resource.

It is very important that aio users check the return value and do
not resubmit an operation once NNG_ECLOSED is returned.  Otherwise
a circular loop can result.

src/core/aio.c

@@ -356,7 +356,7 @@ nni_aio_begin(nni_aio *aio)
 		aio->a_sleep     = false;
 		aio->a_expire_ok = false;
 		nni_mtx_unlock(&eq->eq_mtx);
-
+		nni_task_dispatch(&aio->a_task);
 		return (NNG_ECANCELED);
 	}
 	nni_task_prep(&aio->a_task);