Fix race in exit of eta/util thread

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/fio.c b/fio.c
index 77451db..9d9027d 100644
--- a/fio.c
+++ b/fio.c
@@ -135,6 +135,8 @@
 
 	while (threads) {
 		usleep(DISK_UTIL_MSEC * 1000);
+		if (!threads)
+			break;
 		update_io_ticks();
 		print_thread_status();
 	}