Clear f on error get_next_file_rr()

Otherwise we could return an invalid non-open file and trigger and
assertion failure later in the process.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index fc49bd3..1e9018c 100644
--- a/io_u.c
+++ b/io_u.c
@@ -686,6 +686,7 @@
 			if (err) {
 				dprint(FD_FILE, "error %d on open of %s\n",
 					err, f->file_name);
+				f = NULL;
 				continue;
 			}
 			opened = 1;