check if last opened file exists before opening it

This should fix #916.

reader.lua

@@ -134,6 +134,9 @@ if fontmap ~= nil then
 end
 -- last file
 local last_file = G_reader_settings:readSetting("lastfile")
+if last_file and lfs.attributes(last_file, "mode") ~= "file" then
+    last_file = nil
+end
 -- load last opened file
 local open_last = G_reader_settings:readSetting("open_last")
 -- night mode