Insert Lines should be a noop if outside margins

From esctest:
ICHTests.test_ICH_ScrollOffRightMarginInScrollRegion

src/Screen.cpp

@@ -320,6 +320,9 @@ void Screen::deleteLines(int n)
 
 void Screen::insertLines(int n)
 {
+    if (_cuY < _topMargin) {
+        return;
+    }
     if (n < 1) {
         n = 1; // Default
     }