CRM_Mailing_ActionTokens - Degarde gracefully

Most of the `CRM_*_Tokens` classes include a `checkActive()` function whic
prevents them from trying to do anything if they don't have the necessary
data.  However, this one was missing it, which means that it attempts to
evalute `{action.*}` tokens even when they're not valid.

CRM/Mailing/ActionTokens.php

@@ -61,6 +61,13 @@ public function __construct() {
     ));
   }
 
+  /**
+   * @inheritDoc
+   */
+  public function checkActive(\Civi\Token\TokenProcessor $processor) {
+    return !empty($processor->context['mailingId']) || !empty($processor->context['mailing']);
+  }
+
   /**
    * @inheritDoc
    */