Fixed window attach at the end for multi monitor setup

[Rimsoo]

Not resetting c->next with multi monitors creates an endless loop and crashes.

Step to reproduce:

1. Set config static const int new_window_attach_on_end = 1;
2. Open one client in each monitor
3. Move one of the clients to the other monitor
4. Move back a client to the first window
   -> Start crashing => ctrl+alt+f3 to kill chadwm

Fix:

Reset the client chained list:

c->next = NULL