spicec: fix ASSERT to accept size == 0
spicec: fix ASSERT to accept size == 0

which is useful when calling RedClient::on_clipboard_notify(VD_AGENT_CLIPBOARD_NONE, NULL, 0);

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