avoid redraw when there's no change while i was timing the performance issue, i noticed that there was lots of random redrawing going on. turns out there were coming from here; if someone presses CTRL/ALT etc without pressing anything else, nothing will be inserted, so nothing will change. but the code will `break`, go down and do a needless redraw. this patch changes it to simply return if the keypress iscntrl() also avoid potential UB by casting *buf into an unsigned char.