错误兼容

@@ -6,7 +6,7 @@ const model = require('../models/myqrcode'),
 exports.index = (req, res, next) => {
     let params = req.query,
         esc = false,
-        appversion = req.query.app_version;
+        appversion = req.query.app_version || '';
 
     if (Number(appversion.substr(0, 1) < 5) || (Number(appversion.substr(0, 1)) === 5 && Number(appversion.substr(2, 1)) < 2)) {
         esc = true;