fixed CRL check return value on revoked certificates

fixed possible refcounting bugs
generic return_null() implementation

src/charon/plugins/stroke/stroke.c

@@ -281,14 +281,6 @@ static void ca_section_destroy(ca_section_t *this)
 	free(this);
 }
 
-/**
- * another return NULL 
- */
-static void* return_null()
-{
-	return NULL;
-}
-
 /**
  * data to pass to create_inner_cdp
  */