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>