winbindd: verify that client has closed the connection

winbindd: verify that client has closed the connection

A recent change was to remove a client if the client socket
has become readable. In this change, a check is added to
determine the source of the readbility (actual readability,
closed connection, or some other error), and a suitable
debug message is printed.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11397

Signed-off-by: Uri Simchoni <urisimchoni@gmail.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>