new feature entropy filter to ignore gray frame without iframe

opencv-motion-detect/evcloudsvc.cpp

@@ -156,11 +156,11 @@ class HttpSrv{
             try{
                 string sn = req.get_param_value("sn");
                 string module = req.get_param_value("module");             
-                auto cfg = json::parse(req.body);              
                 if(sn.empty()||module.empty()){
                     throw StrException("no para sn/module");
                 }
 
+                auto cfg = json::parse(req.body); 
                 string key, modname;
                 if(module == "evmgr") {
                     key = sn;