Refactor long if block in getItems in listings

[MAILPOET-1537]

assets/js/src/listing/listing.jsx

@@ -456,9 +456,9 @@ const Listing = React.createClass({
     this.initWithParams(params);
   },
   getItems: function getItems() {
-    if (this.isComponentMounted) {
-      this.setState({ loading: true });
+    if (!this.isComponentMounted) return;
 
+    this.setState({ loading: true });
     this.clearSelection();
 
     MailPoet.Ajax.post({
@@ -504,7 +504,6 @@ const Listing = React.createClass({
         );
       }
     });
-    }
   },
   handleRestoreItem: function handleRestoreItem(id) {
     this.setState({