fixed CRL check return value on revoked certificates

fixed possible refcounting bugs
generic return_null() implementation

src/charon/plugins/sql/sql_cred.c

@@ -331,14 +331,6 @@ static enumerator_t* create_shared_enumerator(private_sql_cred_t *this,
 	return &e->public;
 }
 
-/**
- * return null
- */
-static void *return_null()
-{
-	return NULL;
-}
-
 /**
  * Implementation of sql_cred_t.destroy.
  */