fix(gridster): overlapping widget problem

Author: bastianonm

src/jquery.gridster.js

@@ -1734,10 +1734,14 @@
     * @return {Class} Returns the instance of the Gridster Class.
     */
     fn.move_widget_down = function($widget, y_units) {
-        var el_grid_data = $widget.coords().grid;
-        var actual_row = el_grid_data.row;
-        var moved = [];
-        var y_diff = y_units;
+        var el_grid_data, actual_row, moved, y_diff;
+
+        if (y_units <= 0) { return false; }
+
+        el_grid_data = $widget.coords().grid;
+        actual_row = el_grid_data.row;
+        moved = [];
+        y_diff = y_units;
 
         if (!$widget) { return false; }