LSM: Check for NULL cred-security on free

commit a5795fd38ee8194451ba3f281f075301a3696ce2 upstream.

From: Casey Schaufler <casey@schaufler-ca.com>

Check that the cred security blob has been set before trying
to clean it up. There is a case during credential initialization
that could result in this.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <james.morris@microsoft.com>
Reported-by:  <syzbot+69ca07954461f189e808@syzkaller.appspotmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>