Client no longer generates random client ids for v3.1.1 clients

These are now expected to be generated on the broker. This matches the
behaviour for v5 clients.

Closes #291.

ChangeLog.txt

@@ -3,6 +3,11 @@ Broker:
   then try "nobody" instead. This reduces the burden on users installing
   Mosquitto themselves.
 
+Client library:
+- Client no longer generates random client ids for v3.1.1 clients, these are
+  now expected to be generated on the broker. This matches the behaviour for
+  v5 clients. Closes #291.
+
 Clients:
 - Add timeout return code (27) for `mosquitto_sub -W <secs>` and
   `mosquitto_rr -W <secs>`. Closes #275.

lib/connect.c

@@ -44,7 +44,8 @@ static int mosquitto__connect_init(struct mosquitto *mosq, const char *host, int
 	if(!mosq) return MOSQ_ERR_INVAL;
 	if(!host || port <= 0) return MOSQ_ERR_INVAL;
 
-	if(mosq->id == NULL && (mosq->protocol == mosq_p_mqtt31 || mosq->protocol == mosq_p_mqtt311)){
+	/* Only MQTT v3.1 requires a client id to be sent */
+	if(mosq->id == NULL && (mosq->protocol == mosq_p_mqtt31)){
 		mosq->id = (char *)mosquitto__calloc(24, sizeof(char));
 		if(!mosq->id){
 			return MOSQ_ERR_NOMEM;