CVE-2024-42255 tpm: Use auth only after NULL check in tpm_buf_check_hmac_response()

In the Linux kernel, the following vulnerability has been resolved:

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.]