eCryptfs: ecryptfs_keyring_auth_tok_for_sig() bug fix

The pointer '(*auth_tok_key)' is set to NULL in case request_key()
fails, in order to prevent its use by functions calling
ecryptfs_keyring_auth_tok_for_sig().
Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Cc: <stable@kernel.org>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>

fs/ecryptfs/keystore.c

@@ -1563,6 +1563,7 @@ int ecryptfs_keyring_auth_tok_for_sig(struct key **auth_tok_key,
 		printk(KERN_ERR "Could not find key with description: [%s]\n",
 		       sig);
 		rc = process_request_key_err(PTR_ERR(*auth_tok_key));
+		(*auth_tok_key) = NULL;
 		goto out;
 	}
 	(*auth_tok) = ecryptfs_get_key_payload_data(*auth_tok_key);