fix invalid access in aio notification

issue found and patch provided by Jens Gustedt. after the atomic store
to the error code field of the aiocb, the application is permitted to
free or reuse the storage, so further access is invalid. instead, use
the local copy that was already made.

src/aio/aio_readwrite.c

@@ -51,7 +51,7 @@ static void *io_thread(void *p)
 
 	__aio_wake();
 
-	switch (cb->aio_sigevent.sigev_notify) {
+	switch (sev.sigev_notify) {
 	case SIGEV_SIGNAL:
 		notify_signal(&sev);
 		break;