tomoyo: fix potential use after free

The original code returns a freed pointer.  This function is expected to
return NULL on errors.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>