Add safety check that does not allow empty strings to be queued

to the devctl notification queue. Empty strings cause devctl read
call to return 0 and result in devd exiting prematurely.

The actual offender (ugen notes for root hubs) will be fixed
by separate commit.

sys/kern/subr_bus.c

@@ -532,6 +532,12 @@ devctl_queue_data(char *data)
 	struct dev_event_info *n1 = NULL;
 	struct proc *p;
 
+	/*
+	 * Do not allow empty strings to be queued, as they
+	 * cause devd to exit prematurely.
+	 */
+	if (strlen(data) == 0)
+		return;
 	n1 = malloc(sizeof(*n1), M_BUS, M_NOWAIT);
 	if (n1 == NULL)
 		return;