server: Don't release SndChannel twice from worker reference
server: Don't release SndChannel twice from worker reference

When we release the SndChannel reference during
snd_disconnect_channel(), we need to set the pointer to NULL so it
doesn't get released again on client reconnect during
snd_set_playback_peer(). This can happen when a reference is held from
_playback_get_buffer().

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