Clean this.$triggerPrev on unbind

Author: bordeo

src/extension/trigger.js

@@ -106,6 +106,7 @@ IASTriggerExtension.prototype.bind = function(ias) {
  */
 IASTriggerExtension.prototype.unbind = function(ias) {
   this.$triggerNext = null;
+  this.$triggerPrev = null;
   ias.off('next', this.showTriggerNext);
   ias.off('rendered', this.onRendered);