Added a check to see if the 'speedbar-buffer' is still alive

Fix suggested by Rudi Schlatte <rudi@constantly.at>.
* lisp/speedbar.el (speedbar-frame-or-window): Added an additional check
to see if 'speedbar-buffer' is still alive (bug#77405).

lisp/speedbar.el

@@ -1048,7 +1048,8 @@ Return nil if both are closed."
    ((speedbar-window--live-p)
     'window)
    ((and (frame-live-p (speedbar-current-frame))
-	 speedbar-buffer
+         speedbar-buffer
+	 (buffer-live-p speedbar-buffer)
 	 (not (speedbar-window--live-p)))
     'frame)
    (t nil)))