Don't close the port until after the threads terminate

input_thread will still toggle DTR.

src/syncterm/modem.c

@@ -295,10 +295,10 @@ int modem_connect(struct bbslist *bbs)
 int serial_close(void)
 {
 	conn_api.terminate=1;
-	comClose(com);
 
 	while(conn_api.input_thread_running || conn_api.output_thread_running)
 		SLEEP(1);
+	comClose(com);
 	destroy_conn_buf(&conn_inbuf);
 	destroy_conn_buf(&conn_outbuf);
 	FREE_AND_NULL(conn_api.rd_buf);