Fixes "undefined offset" error when moving a many_many item to previous page (#121)

When viewing the last page of a list of data objects, trying to move one of them to the previous page triggers an "undefined offset" error.

src/Forms/GridFieldSortableRows.php

@@ -656,7 +656,7 @@ class GridFieldSortableRows implements GridField_HTMLProvider, GridField_ActionP
 
             $i = 1;
             foreach ($items as $obj) {
-                if ($obj->ID == $targetItem->ID) {
+                if ($obj->ID == $targetItem->ID || $i >= count($sortPositions)) {
                     continue;
                 }