sd-dhcp-client: avoid "maybe-uninitialized" warning in client_timeout_resend()

When compiling with CFLAGS='-Werror=maybe-uninitialized -Og' we get a
warning about uninitialized "next_timeout" variable.

Avoid the warning by adding an (unreachable) "default" label.

Fixes: c24288d21e ("sd-dhcp-client: correct dhcpv4 renew/rebind retransmit timeouts")

src/libsystemd-network/sd-dhcp-client.c

@@ -1275,6 +1275,9 @@ static int client_timeout_resend(
         case DHCP_STATE_STOPPED:
                 r = -EINVAL;
                 goto error;
+
+        default:
+                assert_not_reached("Unhandled choice");
         }
 
         r = event_reset_time(client->event, &client->timeout_resend,