check if acerts linked_list has been initialized before destroying it

src/pluto/ac.c

@@ -45,9 +45,12 @@ void ac_initialize(void)
  * Free the linked list of attribute certificates
  */
 void ac_finalize(void)
+{
+	if (acerts)
 	{
 		acerts->destroy_offset(acerts, offsetof(certificate_t, destroy));
 	}
+}
 
 /**
  *  Get a X.509 attribute certificate for a given holder