trusted-keys: check for NULL before using it

TSS_rawhmac() checks for data != NULL before using it.
We should do the same thing for TSS_authhmac().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>