unflushed_path_cache: check for nil channel before closing
unflushed_path_cache: check for nil channel before closing

Issue: KBFS-3101

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