fix flake in EncryptedSuite.TestTamperedRoundtrip

If we get really unlucky the encrypted bytes already start with 0x0000
so the tampering fails.

Change-Id: I66581683e1bb1548a3ac87dd8a499be02800ee1a
diff --git a/encrypted/encrypted_test.go b/encrypted/encrypted_test.go
index 31b0582..b9ab9c5 100644
--- a/encrypted/encrypted_test.go
+++ b/encrypted/encrypted_test.go
@@ -44,8 +44,7 @@
 	err = json.Unmarshal(enc, data)
 	c.Assert(err, IsNil)
 
-	data.Ciphertext[0] = 0
-	data.Ciphertext[1] = 0
+	data.Ciphertext[0] = ^data.Ciphertext[0]
 
 	enc, _ = json.Marshal(data)