Fix double io_u free in error path

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index fead400..e96d495 100644
--- a/io_u.c
+++ b/io_u.c
@@ -487,7 +487,7 @@
 
 			if (!f || (ret = td_io_open_file(td, f))) {
 				put_io_u(td, io_u);
-				break;
+				return NULL;
 			}
 			goto set_file;
 		}