rpc_address::ipv4_from_host() returns 0 if resolve host failed

[qinzuoyan]

and also, rpc_address::ipv4_from_network_interface() returns 0 if no proper IP address found.

so that, rpc_address::assign_ipv4() and rpc_address::assign_ipv4_local_address() may assign unexpected ip(0.0.0.0), which may cause tricky problems.

need fix it.