handle first time desk setting for fullscreen clients without crashing

ref T5988

src/bin/e_client.c

@@ -2832,6 +2832,7 @@ e_client_desk_set(E_Client *ec, E_Desk *desk)
 
    if (ec->fullscreen)
      {
+        if (ec->desk)
           ec->desk->fullscreen_clients = eina_list_remove(ec->desk->fullscreen_clients, ec);
         desk->fullscreen_clients = eina_list_append(desk->fullscreen_clients, ec);
      }