cmd/ctr: fix export command

Signed-off-by: Iceber Gu <wei.cai-nat@daocloud.io>

cmd/ctr/commands/images/export.go

@@ -111,7 +111,7 @@ When '--all-platforms' is given all images in a manifest list must be available.
 		} else {
 			w, err = os.Create(out)
 			if err != nil {
-				return nil
+				return err
 			}
 		}
 		defer w.Close()