server/reds: protect reds_update_mouse_mode when main_channel is disconnected
server/reds: protect reds_update_mouse_mode when main_channel is disconnected

ya-mouse/spice,dezelin/spice,Fantu/spice,mathslinux/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,alon/spice-server,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,dezelin/spice,ya-mouse/spice,Fantu/spice,Fantu/spice,mathslinux/spice,alon/spice-server,mathslinux/spice,Fantu/spice,fgouget/spice,alon/spice-server,elmarco/spice,elmarco/spice,ya-mouse/spice,SPICE/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,dezelin/spice,dezelin/spice,fgouget/spice,alon/spice-server,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,SPICE/spice,mathslinux/spice,dezelin/spice,mathslinux/spice