favorites: fix shutdown race when buffered channel triggers first
favorites: fix shutdown race when buffered channel triggers first

We don't want to return immediately if we notice the buffered channel
is closed, because it will prevent us from calling `f.wg.Done()` for
the requests queued up in the regular, non-buffered channel.

Issue: HOTPOT-73

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client