[fix] MenuSorter: don't leave ghost top level items (#3844)

See https://github.com/koreader/koreader/pull/3773#issuecomment-379181517

frontend/ui/menusorter.lua

@@ -131,12 +131,14 @@ function MenuSorter:sort(item_table, order)
     end
     -- cleanup, top-level items shouldn't have sub_item_table
     -- they should, however have one going in
+    -- Also, compress the menu table.
     local menu_buttons_offset = 0
     for i,top_menu in ipairs(menu_table["KOMenu:menu_buttons"]) do
-        if menu_table["KOMenu:menu_buttons"][i].sub_item_table then
-            menu_table["KOMenu:menu_buttons"][i-menu_buttons_offset] = menu_table["KOMenu:menu_buttons"][i].sub_item_table
+        local menu_button = menu_table["KOMenu:menu_buttons"][i].sub_item_table
+        menu_table["KOMenu:menu_buttons"][i] = nil
+        if menu_button then
+            menu_table["KOMenu:menu_buttons"][i-menu_buttons_offset] = menu_button
         else
-            menu_table["KOMenu:menu_buttons"][i] = nil
             menu_buttons_offset = menu_buttons_offset + 1
         end
     end