Fixed XmuClientWindow, was killing Perl with BadWindow

I finally tracked down the cause of a BadWindow that was unceremoniously killing our application.

I still don't know what is sending the ClientMessage(WM_PROTOCOL) to us or how to provoke that,
but it gives a windowid=0x1 and the would crash the app until d2446dd.

I haven't attempted to test that XmuClientWindow still works, but I have seen ClientMessages go past
which would otherwise have crashed.