return statement to exit reading loop

Merges PR#2414

I used this example to start building my WebSocket application, however, I noticed that upon removing the echoing in the on_read method, the server repeatedly throws errors without exiting the method.

You can recreate this problem by replacing the async_write with do_write and cause any error (i.e. client reloads the page).

Adding a return statement here would make debugging for future changes much easier and it wouldn't harm the existing echoing in any way.

example/websocket/server/async/websocket_server_async.cpp

@@ -126,7 +126,7 @@ public:
             return;
 
         if(ec)
-            fail(ec, "read");
+            return fail(ec, "read");
 
         // Echo the message
         ws_.text(ws_.got_text());