[PATCH] Fix double free in sg engine error path

We need to clear td->io_ops->data if we free the structure,
otherwise the ->cleanup() handler will try to free it again.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/fio-engine-sg.c b/engines/fio-engine-sg.c
index e074889..3ea1e28 100644
--- a/engines/fio-engine-sg.c
+++ b/engines/fio-engine-sg.c
@@ -311,6 +311,7 @@
 	free(sd->events);
 	free(sd->cmds);
 	free(sd);
+	td->io_ops->data = NULL;
 	return 1;
 }