Examples clear the HTTP message before reading

fix #1043

example/advanced/server-flex/advanced_server_flex.cpp

@@ -787,6 +787,10 @@ public:
         // Set the timer
         timer_.expires_after(std::chrono::seconds(15));
 
+        // Make the request empty before reading,
+        // otherwise the operation behavior is undefined.
+        req_ = {};
+
         // Read a request
         http::async_read(
             derived().stream(),