fix walk

src/vfs/utils.go

@@ -143,8 +143,12 @@ func readdirnames(ctx context.Context, vfs Filesystem, dir string) ([]string, er
 		return nil, err
 	}
 
-	var names []string
+	names := make([]string, 0, len(files))
 	for _, file := range files {
+		if file == nil {
+			continue
+		}
+
 		names = append(names, file.Name())
 	}