folder_branch_ops: avoid panic during resets when there's no MD
folder_branch_ops: avoid panic during resets when there's no MD

If the MD for a TLF is completely unreadable from the mdserver, we
should still allow a reset.

Issue: HOTPOT-954

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