Fix for alvarotrigo#3931 (normalScrollElements option)

There is a fix for the issue alvarotrigo#3931, that appears when "normalScrollElements" option is added and mouse cursor leaves from a browser window.

When the mouse cursor left the viewport, the "scrollable" variable was not assigned a value, because the "target" parameter, in this case, is null. So, I just added a check for the "target" parameter value in 2409 line.

vendors/scrolloverflow.js

@@ -2406,7 +2406,7 @@ if ( typeof module != 'undefined' && module.exports ) {
 
             // Enables or disables the whole iScroll feature based on the given parameter.
             setIscroll: function(target, enable){
-                if(!iscrollHandler.hasBeenInit){
+                if(!iscrollHandler.hasBeenInit || !target){
                     return;
                 }
                 var scrollable = fp_utils.closest(target, SCROLLABLE_SEL) || $(SCROLLABLE_SEL, target)[0];
@@ -2599,4 +2599,4 @@ if ( typeof module != 'undefined' && module.exports ) {
             iscrollHandler: iscrollHandler
         };
     })();
-})(window, document);
+})(window, document);