Use BETWEEN in tinsertblankline and tdeleteline.

st.c

@@ -1628,17 +1628,13 @@ tinsertblank(int n) {
 
 void
 tinsertblankline(int n) {
-	if(term.c.y < term.top || term.c.y > term.bot)
+	if(BETWEEN(term.c.y, term.top, term.bot))
-		return;
-
 		tscrolldown(term.c.y, n);
 }
 
 void
 tdeleteline(int n) {
-	if(term.c.y < term.top || term.c.y > term.bot)
+	if(BETWEEN(term.c.y, term.top, term.bot))
-		return;
-
 		tscrollup(term.c.y, n);
 }