hw/nxagent/Events.c: Only declare and assign message_data and box variables if compiled with -DTEST / -DDEBUG (plus drop commented out code).

 Amends the following compiler warnings:

 ```
 Events.c: In function ‘nxagentHandleClientMessageEvent’:
 Events.c:2694:13: warning: variable ‘message_data’ set but not used [-Wunused-but-set-variable]
        char *message_data;
              ^
 Events.c: In function ‘nxagentClipAndSendExpose’:
 Events.c:4313:10: warning: variable ‘box’ set but not used [-Wunused-but-set-variable]
    BoxRec box;
           ^
 ```