Remove unsigned comparison against <0.

Coverity CID 1292471.

src/conf.c

@@ -1424,7 +1424,9 @@ int _config_read_file_core(struct mqtt3_config *config, bool reload, const char
 					}
 				}else if(!strcmp(token, "message_size_limit")){
 					if(_conf_parse_int(&token, "message_size_limit", (int *)&config->message_size_limit, saveptr)) return MOSQ_ERR_INVAL;
-					if(config->message_size_limit < 0 || config->message_size_limit > MQTT_MAX_PAYLOAD){
+					if(config->message_size_limit > MQTT_MAX_PAYLOAD){
+						y
+						l
 						_mosquitto_log_printf(NULL, MOSQ_LOG_ERR, "Error: Invalid message_size_limit value (%d).", config->message_size_limit);
 						return MOSQ_ERR_INVAL;
 					}