Fix undefined reference to event

Signed-off-by: James Ketrenos <james_git@ketrenos.com>

frontend/src/ketr-photos/ketr-photos.html

@@ -428,7 +428,7 @@
 
         onScroll: function(event) {
           if (this.disableScrolling) {
-            event.preventDefault();
+            if (event) event.preventDefault();
             window.scrollTo(this.topStickX, this.topStickY);
             return;
           }