clamp scroll

st.c

@@ -1091,6 +1091,9 @@ void
 kscrollup(const Arg* a)
 {
 	int n = a->i;
+    int maxup = term.histi - term.scr;
+    if (maxup <= 0) return;
+    if (n > maxup) n = maxup;
 
 	if (n < 0)
 		n = term.row + n;