Fix of NULL-pointer dereference segfault in texinfo description.


  The problem is due missing variable initialization in the following code snip in session.c:

   /* Use KEY (a digit) to select the Nth menu item in WINDOW->node. */
   DECLARE_INFO_COMMAND (info_menu_digit, _("Select this menu item"))
   {
     register int i, item;
[1]  register REFERENCE *entry = NULL, **menu;

     menu = info_menu_of_node (window->node);

     if (!menu)
       {
         info_error ((char *) msg_no_menu_node, NULL, NULL);
         return;
       }

     /* We have the menu.  See if there are this many items in it. */
     item = key - '0';

     /* Special case.  Item "0" is the last item in this menu. */
[2]  if (item == 0)
[3]    for (i = 0; menu[i + 1]; i++);
     else
       {
         for (i = 0; (entry = menu[i]); i++)
           if (i == item - 1)
             break;
       }

[4]  if (menu[i])
       {
         info_select_reference (window, menu[i]);
[5]      if (entry->line_number > 0)
...

    As we can see there is a case when item==0 [2] and menu[i]!=NULL [4]. In this case segfault occurs on the NULL
 pointer dereference of variable 'entry' at [5], because value of the 'entry' variable still has its initial value of NULL[1].
    According to texinfo when '0' key pressed(item=0) we should move to the last menu item available. So the 'entry'
 should be initialized in this case and it should point to the last menu item that is last menu[i] pointer before NULL. 
    I assume following solution and some code cleanup:

  if (item == 0)
    for (i = 0; menu[i + 1]; i++);
  else
    {
      for (i = 0; menu[i]; i++)
        if (i == item - 1)
          break;
    }

  entry=menu[i];

  if (entry)
    {
      info_select_reference (window, entry);
      if (entry->line_number > 0)
        info_next_line (window, entry->line_number - 1, key);
    }