Fix two bugs that were stopping net rpc vampire from working over secure

Fix two bugs that were stopping net rpc vampire from working over secure
channel:

  - If the domain name passed to create_rpc_bind_req() is empty, use
    lp_workgroup()

  - Correctly set the auth_padding field when the send_size is a multiple
    of 8 bytes

I've tested with nt4sp6 and win2ksp0 and it seems to work, although
there are no password hashes transferred from win2k.  The empty
passwords are being protected by the secure channel encryption though.
(This used to be commit a8c11e855611c91e94787387c62ac629232cacfa)