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.

dmenu.c

@@ -415,8 +415,9 @@ keypress(XKeyEvent *ev)
 	switch(ksym) {
 	default:
 insert:
-		if (!iscntrl(*buf))
-			insert(buf, len);
+		if (iscntrl((unsigned char)*buf))
+			return;
+		insert(buf, len);
 		break;
 	case XK_Delete:
 	case XK_KP_Delete: