[191] Fix some $SYS messages being incorrectly persisted.

Closes #191.

Bug: https://github.com/eclipse/mosquitto/issues/191

ChangeLog.txt

@@ -5,6 +5,7 @@ Broker:
 - Fix TLS operation with websockets listeners and libwebsockts 2.x. Closes
   #186.
 - Don't disconnect client on HUP before reading the pending data. Closes #7.
+- Fix some $SYS messages being incorrectly persisted. Closes #191.
 
 Build:
 - Don't attempt to install docs when WITH_DOCS=no. Closes #184.

src/persist.c

@@ -140,7 +140,7 @@ static int mqtt3_db_message_store_write(struct mosquitto_db *db, FILE *db_fptr)
 	stored = db->msg_store;
 	while(stored){
 		if(stored->topic && !strncmp(stored->topic, "$SYS", 4)){
-			if(stored->ref_count == 1 && stored->dest_id_count == 0){
+			if(stored->ref_count <= 1 && stored->dest_id_count == 0){
 				/* $SYS messages that are only retained shouldn't be persisted. */
 				stored = stored->next;
 				continue;