Protect vsl avoid called error.

Author: tangbc

examples/build/finite.js

@@ -90,7 +90,7 @@
         methods: {
             onScroll: function (e) {
                 var delta = this.delta
-                var offset = this.$refs.vsl.scrollTop
+                var offset = (this.$refs.vsl && this.$refs.vsl.scrollTop) || 0
 
                 if (delta.total > delta.keeps) {
                     this.updateZone(offset)

examples/build/infinite.js

@@ -1,6 +1,6 @@
 {
   "name": "vue-virtual-scroll-list",
-  "version": "1.2.6",
+  "version": "1.2.7",
   "description": "A vue (2.x) component that support big data list with high scroll performance.",
   "main": "index.js",
   "files": [