A ->close_file() operation isn't strictly needed

So be sure to check the pointer before calling it.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/ioengines.c b/ioengines.c
index 998c13e..b9c6d71 100644
--- a/ioengines.c
+++ b/ioengines.c
@@ -305,7 +305,8 @@
 	get_file(f);
 	return 0;
 err:
-	td->io_ops->close_file(td, f);
+	if (td->io_ops->close_file)
+		td->io_ops->close_file(td, f);
 	return 1;
 }
 
@@ -318,4 +319,3 @@
 
 	put_file(td, f);
 }
-