macaroons: don't zero mem if never unlocked

macaroons/store.go

@@ -196,6 +196,8 @@ func (r *RootKeyStorage) RootKey(_ context.Context) ([]byte, []byte, error) {
 // Close closes the underlying database and zeroes the encryption key stored
 // in memory.
 func (r *RootKeyStorage) Close() error {
-	r.encKey.Zero()
+	if r.encKey != nil {
+		r.encKey.Zero()
+	}
 	return r.DB.Close()
 }