Reuse result of previous calcualtion

This closes https://github.com/select2/select2/pull/4155.

src/js/select2/results.js

@@ -397,11 +397,7 @@ define([
       this.$results.on('mousewheel', function (e) {
         var top = self.$results.scrollTop();
 
-        var bottom = (
-          self.$results.get(0).scrollHeight -
-          self.$results.scrollTop() +
-          e.deltaY
-        );
+        var bottom = self.$results.get(0).scrollHeight - top + e.deltaY;
 
         var isAtTop = e.deltaY > 0 && top - e.deltaY <= 0;
         var isAtBottom = e.deltaY < 0 && bottom <= self.$results.height();