History: avoid opening non-existent files

Mostly just so we don't get the History closed when
erroneously tapping on a deleted file.

frontend/readhistory.lua

@@ -39,6 +39,9 @@ local function buildEntry(input_time, input_file)
             end
             return util.secondsToDate(last_read_ts, G_reader_settings:isTrue("twelve_hour_clock"))
         end,
+        select_enabled_func = function()
+            return lfs.attributes(file_path, "mode") == "file"
+        end,
         callback = function()
             local ReaderUI = require("apps/reader/readerui")
             ReaderUI:showReader(input_file)

frontend/ui/widget/menu.lua

@@ -1173,6 +1173,14 @@ override this function to process the item selected in a different manner
 ]]--
 function Menu:onMenuSelect(item)
     if item.sub_item_table == nil then
+        if item.select_enabled == false then
+            return true
+        end
+        if item.select_enabled_func then
+            if not item.select_enabled_func() then
+                return true
+            end
+        end
         if self.close_callback then
             self.close_callback()
         end