desk_add: fix starting desk/monitor

When moving windows between screens using the keyboard, the mouse
pointer won't be on the same screen as the window is.   Rather than make
that assumption, look up the desk of the monitor the window is currently
on.

fvwm/virtual.c

@@ -2897,11 +2897,13 @@ desk_add_fw(FvwmWindow *fw)
 {
 	struct desktop_fw	*df, *df_loop;
 	struct desk_fvwmwin	*dfws;
-	int			 desk = monitor_get_current()->virtual_scr.CurrentDesk;
+	int			 desk;
 
 	if (fw == NULL)
 		return;
 
+	desk = fw->m->virtual_scr.CurrentDesk;
+
 	if (TAILQ_EMPTY(&desktop_fvwm_q))
 		TAILQ_INIT(&desktop_fvwm_q);