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: c24288d2 ("sd-dhcp-client: correct dhcpv4 renew/rebind retransmit timeouts")