common/pick_address.cc: Copy public_netw to cluset_netw if cluster empty

common/pick_address.cc: Copy public_netw to cluset_netw if cluster empty

 - When public network is set, but cluster network is not, then
   the cluster-bindings would be on 0.0.0.0 which could be unexpeted.

 In this commit we copy the public network into the cluster network
 to make sure that the cluster backend is not bound on 0.0.0.0
 Which could be consideren an insecure, or unexpected, action.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>