examples/dhcp_client.rs is misleading/wrong

[pdh11]

As of 0.9.1 and current main 8b0c521, the "dhcp_client.rs" example, when it gets a Configured event, attempts to set the interface's IPv4 address as follows:

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);
    });
}

This doesn't actually work in Smoltcp 0.9.1, because iter_mut() on a heapless::Vec iterates only over the existing items, and is incapable of adding one if the Vec is currently empty. It always panics unwrapping the error from next(). This code worked in 0.8.x because iter_mut() on a mutable slice (which is what ManagedSlice derefs to) does iterate over all elements (which were set to UNSPECIFIED if not currently in use).

For smoltcp 0.9.x, set_ipv4_addr should probably do something like:

                iface.update_ip_addrs(|addrs| {
                    addrs.push(IpCidr::Ipv4(config.address)).unwrap();
                });

and likewise on a Deconfigured event it should do addrs.clear().

[jlogan03]

@pdh11 thanks for making this issue! This probably saved me another 6 hours of troubleshooting while thinking this was a problem with my network switch. I made #905 to fix this, and have a piece of firmware based on those fixes running on a board now w/ IP address acquired successfully.