Do not try to set up default gateway without an IP address

If the container has no IP address (because it will be assigned via DHCP later),
setting up a default route fails with a warning message.

While this is harmless, the default route should be omitted instead.

src/firejail/network_main.c

@@ -246,6 +246,10 @@ void net_check_cfg(void) {
  if (cfg.defaultgw)
  check_default_gw(cfg.defaultgw);
  else {
+ // if the first network has no assigned address,
+ // do not try to set up a gateway, because it will fail
+ if (cfg.bridge0.arg_ip_none)
+ return;
  // first network is a regular bridge
  if (cfg.bridge0.macvlan == 0)
  cfg.defaultgw = cfg.bridge0.ip;