Fix infinite loop in get_next_file_rr()

We need to clear f to NULL before doing a continue on FIO_FILE_DONE.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 5f51a00..691f7af 100644
--- a/io_u.c
+++ b/io_u.c
@@ -446,8 +446,10 @@
 		if (td->next_file >= td->o.nr_files)
 			td->next_file = 0;
 
-		if (f->flags & FIO_FILE_DONE)
+		if (f->flags & FIO_FILE_DONE) {
+			f = NULL;
 			continue;
+		}
 
 		if ((!goodf || (f->flags & goodf)) && !(f->flags & badf))
 			break;