Fix `max_keepalive` option not applying to keepalive=0.

Closes #2117. Thanks to David Nadlinger.

ChangeLog.txt

@@ -22,6 +22,8 @@ Broker:
 - Fix bridge increasing backoff calculation.
 - Improve handling of invalid combinations of listener address and bind
   interface configurations. Closes #2081.
+- Fix `max_keepalive` option not applying to clients connecting with keepalive
+  set to 0. Closes #2117.
 
 Client library:
 - Fix encrypted connections incorrectly connecting when the CA file passed to

src/handle_connect.c

@@ -244,7 +244,9 @@ int connect__on_authorised(struct mosquitto *context, void *auth_data_out, uint1
 				goto error;
 			}
 		}
-		if(context->keepalive > db.config->max_keepalive){
+		if(db.config->max_keepalive &&
+				(context->keepalive > db.config->max_keepalive || context->keepalive == 0)){
+
 			context->keepalive = db.config->max_keepalive;
 			if(mosquitto_property_add_int16(&connack_props, MQTT_PROP_SERVER_KEEP_ALIVE, context->keepalive)){
 				rc = MOSQ_ERR_NOMEM;