levin: fix wrong value passed as return code to remote call

When receiving an answer packet, the command code was passed
to the callback instead of the error code. This was hiding
the "command not found" failure from the peer, and in turn
causing the code to attempt to deserialize a non existent
reply string.

contrib/epee/include/net/levin_protocol_handler_async.h

@@ -373,7 +373,7 @@ public:
               invoke_response_handlers_guard.unlock();
 
               if(timer_cancelled)
-                response_handler->handle(m_current_head.m_command, buff_to_invoke, m_connection_context);
+                response_handler->handle(m_current_head.m_return_code, buff_to_invoke, m_connection_context);
             }
             else
             {