avoid redraw when there's no change

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.