[fdio] don't try to close non-remote vnodes

Change-Id: Id7055f13f761319a67122ce64c74d56615042b95
diff --git a/src/syscall/zx/fdio/namespace.go b/src/syscall/zx/fdio/namespace.go
index 576e41d..135a6e2 100644
--- a/src/syscall/zx/fdio/namespace.go
+++ b/src/syscall/zx/fdio/namespace.go
@@ -138,6 +138,9 @@
 
 // Close closes the dir.
 func (d *dir) Close() error {
+	if !d.vn.remote.IsValid() {
+		return nil
+	}
 	return d.vn.remote.Close()
 }