[AC21.4] TTY --memory-usage-stats fix.

APPROVE COMMIT RECOMMEND 21.4 Way back in October Zajcev Evgeny reported the following: If XEmacs is compiled with memory usage enabled, running in console and issuing `(window-memory-usage (selected-window))' will core dump XEmacs because tty console does not have method to compute scrollbar instance usage and there no check for that. His suggested solution, cast as a patch below, fixes the problem. This patch has been committed. It is applicable for 21.4 as well. Thanks Zajcev! Malcolm src/ChangeLog addition: 2005-01-06 Zajcev Evgeny <zevlg(a)yandex.ru&gt; * scrollbar.c (compute_scrollbar_instance_usage): Only call device method compute_scrollbar_instance_usage() if it is defined.