Fix QoS 0 messages not being delivered if max_queued_messages was set to 0.

Closes #1956. Thanks to nduhme.

ChangeLog.txt

@@ -5,6 +5,8 @@ Broker:
   being continued. Closes #1946.
 - Fix bridges possibly not completing connections when WITH_ADNS is in use.
   Closes #1960.
+- Fix QoS 0 messages not being delivered if max_queued_messages was set to 0.
+  Closes #1956.
 
 Clients:
 - Fix mosquitto_sub being unable to terminate with Ctrl-C if a successful

src/database.c

@@ -50,6 +50,9 @@ bool db__ready_for_flight(struct mosquitto_msg_data *msgs, int qos)
 		 * There is no queueing option, unless the client is offline and
 		 * queue_qos0_messages is enabled.
 		 */
+		if(db.config->max_queued_messages == 0 && db.config->max_inflight_bytes == 0){
+			return true;
+		}
 		valid_bytes = msgs->msg_bytes - db.config->max_inflight_bytes < db.config->max_queued_bytes;
 		valid_count = msgs->msg_count - msgs->inflight_maximum < db.config->max_queued_messages;