Remove warning when removing display
Some display have no associated window (for ex, if it doesn't fit
on client monitors).
(remote-viewer:22275): remote-viewer-CRITICAL **: virt_viewer_window_set_display: assertion `VIRT_VIEWER_IS_WINDOW(self)' failed
(remote-viewer:22275): remote-viewer-CRITICAL **: virt_viewer_app_remove_nth_window: assertion `win != NULL' failed
https://bugzilla.redhat.com/show_bug.cgi?id=1107518