server/red_worker: turn critical (assert) non error into warning
server/red_worker: turn critical (assert) non error into warning

The situation causing this assert is unknown but it doesn't cause
correctness issues with later rendering, and it is causing an abort.

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