sg engine: do type check only when sd has been setup

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/sg.c b/engines/sg.c
index 44e151b..3462688 100644
--- a/engines/sg.c
+++ b/engines/sg.c
@@ -390,7 +390,7 @@
 	if (ret)
 		return ret;
 
-	if (!sd->type_checked && fio_sgio_type_check(td, f)) {
+	if (sd && !sd->type_checked && fio_sgio_type_check(td, f)) {
 		generic_close_file(td, f);
 		return 1;
 	}