ch_free: deal with rings that have no entries
ch_free: deal with rings that have no entries

it may happen that a ring was constructed, but no servers were added
(due to failure), so don't crash when trying to clean that up

piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,grobian/carbon-c-relay,grobian/carbon-c-relay,grobian/carbon-c-relay