Don't save corrupted NickServ credentials

soju saved most NickServ messages[0] as credentials because of a missing
`default` clause in the check of the NickServ command.

[0] messages that had at least a command and two other parameters

downstream.go

@@ -1682,6 +1682,8 @@ func parseNickServCredentials(text, nick string) (username, password string, ok
 			username = nick
 			password = params[1]
 		}
+	default:
+		return "", "", false
 	}
 	return username, password, true
 }