Fix for php backward incompatible changes

Newer versions of php would crash of this with the error "objects cannot be assigned by reference."

fields/field.xml.php

@@ -34,7 +34,7 @@ public function checkPostFieldData($data, &$message, $entry_id = null)
         }
 
         include_once(TOOLKIT . '/class.xsltprocess.php');
-        $xsltProc =& new XsltProcess;
+        $xsltProc = new XsltProcess();
 
         if (!General::validateXML($data, $errors, false, $xsltProc)) {