Don't continue working with an invalid epub

It produces a seg fault

lib/upload.go

@@ -46,6 +46,7 @@ func (req uploadRequest) processFile(db database.DB, store storage.Store) {
 	if err != nil {
 		log.Warn("Not valid epub uploaded file ", req.header.Filename, ": ", err)
 		db.UpdateSubmission(req.id, "It is not a valid epub file", nil)
+		return
 	}
 	defer epub.Close()