An assertion in dhcp_network_bind_raw_socket() is triggered when
starting an sd_dhcp_client without setting setting a MAC address
first.

  - sd_dhcp_client_start()
    - client_start()
      - client_start_delayed()
        - dhcp_network_bind_raw_socket()

In that case, the arp-type and MAC address is still unset. Note that
dhcp_network_bind_raw_socket() already checks for a valid arp-type
and MAC address below, so we should just gracefully return -EINVAL.

Maybe sd_dhcp_client_start() should fail earlier when starting without
MAC address. But the failure here will be correctly propagated and
the start aborted.

See-also: https://github.com/systemd/systemd/pull/10054
(cherry picked from commit 34af574d)
(cherry picked from commit 0a797bdc)
(cherry picked from commit f37ed84c)