Protect against per_listener_settings being unset after it is set.

src/conf.c

@@ -2035,6 +2035,10 @@ static int config__read_file_core(struct mosquitto__config *config, bool reload,
 					}
 					if(conf__parse_string(&token, "password_file", &cur_security_options->password_file, &saveptr)) return MOSQ_ERR_INVAL;
 				}else if(!strcmp(token, "per_listener_settings")){
+					if(config->per_listener_settings){
+						/* Once this is set, don't let it be unset. It should be the first config option ideally. */
+						continue;
+					}
 					if(conf__parse_bool(&token, "per_listener_settings", &config->per_listener_settings, &saveptr)) return MOSQ_ERR_INVAL;
 					if(cur_security_options && config->per_listener_settings){
 						log__printf(NULL, MOSQ_LOG_ERR, "Error: per_listener_settings must be set before any other security settings.");