Fix client id validation in MQTT 3.1

Motivation:

In MQTT 3.1 specification, "The Client Identifier (Client ID) is between
1 and 23 characters long, and uniquely identifies the client to the
server". But, current check range is 0~23 instead of 1~23.

Modifications:

Update isValidClientId method.

Result:

It'll be close to the specification.