Detect remote disconnect

[Emiliano Gonzalez <egonzalez.hiperion@xxxxxxxxx>]

I have a minimal sshd server and I need to detect if a remote client not
exist any more.

If I connect and kill remote ssh, ssh_log_function (setted by
ssh_set_log_callback) receives userdata == NULL, but what is the correct
form to detect this case?