DHCP: Fail link_dhcp_set_routes promotely if no address is assigned from lease (...

DHCP: Fail link_dhcp_set_routes promotely if no address is assigned from lease (#6009)

Currently the local variable `address` is unintialized if the DHCP lease
doesn't provide a router address (when r == -ENODATA). Thus the
subsequent call to route_scope_from_address will result in accessing an
unintialized variable.

As a matter of fact, sd-dhcp-client ignores DHCP leases without an
address so link_dhcp_set_routes probably will never be called without a
valid address.