tpm: Use auth only after NULL check in tpm_buf_check_hmac_response()

Dereference auth after NULL check in tpm_buf_check_hmac_response().
Otherwise, unless tpm2_sessions_init() was called, a call can cause NULL
dereference, when TCG_TPM2_HMAC is enabled.

[jarkko: adjusted the commit message.]
Cc: stable@vger.kernel.org # v6.10+
Fixes: 7ca110f2

 ("tpm: Address !chip->auth in tpm_buf_append_hmac_session*()")
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>