tpm: Address !chip->auth in tpm2_*_auth_session()

Unless tpm_chip_bootstrap() was called by the driver, !chip->auth can cause
a null derefence in tpm2_*_auth_session(). Thus, address !chip->auth in
tpm2_*_auth_session().

Cc: [e-mail] # v6.9+
Reported-by: Stefan Berger <[e-mail]>
Closes: https://lore.kernel.org/linux-integrity/[e-mail]/
Fixes: 699e3efd6c64 ("tpm: Add HMAC session start and end functions")
Tested-by: Michael Ellerman <[e-mail]> # ppc
Signed-off-by: Jarkko Sakkinen <[e-mail]>

drivers/char/tpm/tpm2-sessions.c

···
  */
 void tpm2_end_auth_session(struct tpm_chip *chip)
 {
-	tpm2_flush_context(chip, chip->auth->handle);
-	memzero_explicit(chip->auth, sizeof(*chip->auth));
+	struct tpm2_auth *auth = chip->auth;
+
+	if (!auth)
+		return;
+
+	tpm2_flush_context(chip, auth->handle);
+	memzero_explicit(auth, sizeof(*auth));
 }
 EXPORT_SYMBOL(tpm2_end_auth_session);
 
···
 	struct tpm2_auth *auth = chip->auth;
 	int rc;
 	u32 null_key;
+
+	if (!auth) {
+		dev_warn_once(&chip->dev, "auth session is not active\n");
+		return 0;
+	}
 
 	rc = tpm2_load_null(chip, &null_key);
 	if (rc)