This can happen since we select for events on the root window, and
clients themselves might not select for input, meaning the X server
will bubble up. Just do nothing and ignore the event in this case.

This should hopefully fix some of the

Window manager warning: Log level 8: meta_window_raise: assertion '!window->override_redirect' failed
Window manager warning: Log level 8: meta_window_focus: assertion '!window->override_redirect' failed

spam that people have been seeing.