IKE_SA unique identifier allocation starts at 1. If the counter overflows,
a unique ID of 0 is assigned to an IKE_SA, which may have unclear consequences.
Overflowing the unique ID counter is theoretical for most setups, but on
a Gateway terminating 100'000 tunnels and rekeying the IKE_SA every 60s
overflows the counter after a month uptime. So avoid a 0 unique identifier
by using ref_get_nonzero().