Clean up two declarations of blob_len

- Eliminate double declaration of variable blob_len
- Modify function build_ntlmssp_auth_blob to return error code
  as well as length of the blob.

Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Reviewed-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>