update dhcp example to work in the case where we do not already have an ip address from some other source

Author: jlogan03

examples/dhcp_client.rs

@@ -77,6 +77,7 @@ fn main() {
             }
             Some(dhcpv4::Event::Deconfigured) => {
                 debug!("DHCP lost config!");
+                iface.update_ip_addrs(|addrs| addrs.clear());
                 set_ipv4_addr(&mut iface, Ipv4Cidr::new(Ipv4Address::UNSPECIFIED, 0));
                 iface.routes_mut().remove_default_ipv4_route();
             }
@@ -88,7 +89,6 @@ fn main() {
 
 fn set_ipv4_addr(iface: &mut Interface, cidr: Ipv4Cidr) {
     iface.update_ip_addrs(|addrs| {
-        let dest = addrs.iter_mut().next().unwrap();
-        *dest = IpCidr::Ipv4(cidr);
+        addrs.push(IpCidr::Ipv4(cidr)).unwrap();
     });
 }