winbindd: Fix crash in cm_connect_sam()

winbindd: Fix crash in cm_connect_sam()

Fix segv when talking to parent DC (joined to child domain).

The root cause was

(a) storing the parent domain in the cli_state struct caused
    the NTLMSSP pipe bind to fail which made us fallover to
    the schannel code path
(b) the dcinfo pointer in cm_get_schannel_dcinfo() was returning
    NULL even though the function indicated success.
(This used to be commit 5ce4a2ae6697970ea37d0078a506615b4b7a9a9c)