reds: lookup corresponding channel id
reds: lookup corresponding channel id

In reds_send_link_ack(), lookup the channel with the same id as the link
message.

The bug was found during code review a while ago.

A reproducer bug was later reported:
https://bugzilla.redhat.com/show_bug.cgi?id=1058625

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