spice-widget: Do not invalidate cursor when display is not ready
spice-widget: Do not invalidate cursor when display is not ready

spice_display_get_scaling() gives wrong x, y coordinates and scaling
when the display is not ready. The wrong values cause runtime
warnings when disabling a virt-viewer's window:
Gtk-CRITICAL **: gtk_widget_queue_draw_area: assertion 'width >= 0' failed

fgouget/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk