fix(screensaver): lastfile will be nil for fresh installation

frontend/ui/screensaver.lua

@@ -98,12 +98,13 @@ function Screensaver:show()
     -- first check book cover image
     if KOBO_SCREEN_SAVER_LAST_BOOK then
         local lastfile = G_reader_settings:readSetting("lastfile")
-        local data = DocSettings:open(lastfile)
-        local exclude = data:readSetting("exclude_screensaver")
-        if not exclude then
-            self.suspend_msg = self:getCoverImage(lastfile)
+        if lastfile then
+            local data = DocSettings:open(lastfile)
+            local exclude = data:readSetting("exclude_screensaver")
+            if not exclude then
+                self.suspend_msg = self:getCoverImage(lastfile)
+            end
         end
-
     end
     -- then screensaver directory or file image
     if not self.suspend_msg then