server/red_channel: add empty handle of SPICE_MSGC_DISCONNECTING
server/red_channel: add empty handle of SPICE_MSGC_DISCONNECTING

Simply ignored in red_channel_handle_message

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