Prevent from scrolling past the end of the content

Fixes issue asciimoo#31

wuzz.go

@@ -781,6 +781,10 @@ func scrollView(v *gocui.View, dy int) error {
 	if oy+dy < 0 {
 		dy = -oy
 	}
+	_, err = v.Line(dy)
+	if dy > 0 && err != nil {
+		dy = 0
+	}
 	v.SetOrigin(ox, oy+dy)
 	return nil
 }