org-agenda: Fix handling of indirect buffer and window

* org-agenda.el (org-agenda-tree-to-indirect-buffer): Fix handling of
indirect buffer and window.

Check if the window of last indirect buffer is still live and properly
handle the case of `org-last-indirect-buffer' being nil.

Changes suggested by Dave Abrahams in <m2boow1q9e.fsf@boostpro.com>.

lisp/org-agenda.el

@@ -7280,11 +7280,11 @@ use the dedicated frame)."
   (if (and current-prefix-arg (listp current-prefix-arg))
       (org-agenda-do-tree-to-indirect-buffer)
     (let ((agenda-window (selected-window))
-          (indirect-window (get-buffer-window org-last-indirect-buffer)))
+          (indirect-window (and org-last-indirect-buffer (get-buffer-window org-last-indirect-buffer))))
       (save-window-excursion (org-agenda-do-tree-to-indirect-buffer))
       (unwind-protect
           (progn
-            (unless indirect-window
+            (unless (and indirect-window (window-live-p indirect-window))
               (setq indirect-window (split-window agenda-window)))
             (select-window indirect-window)
             (switch-to-buffer org-last-indirect-buffer :norecord)