sockets: skip NULL ifa_addr entries in get_interfaces instead of stopping

[a-rank]

getifaddrs may return entries with ifa_addr==NULL, for example a MAC-less TUN like tailscale0 or wireguard. The loop used ifa->ifa_addr as its continuation condition, so enumeration stopped at the first such entry returning an empty list.

[Copilot]

Pull request overview

This PR fixes a bug in the Unix implementation of rpp::ipinterface::get_interfaces where network interface enumeration would stop prematurely. The previous code used ifa->ifa_addr as part of the for loop continuation condition, meaning that as soon as getifaddrs returned an entry with ifa_addr == NULL (common for MAC-less TUN interfaces such as tailscale0 or wireguard), the loop terminated. This could result in an empty interface list. Since get_interfaces is a public API that also feeds get_ip_interface, get_system_ip, and get_broadcast_ip, this fix improves reliability of interface discovery on Linux/Unix systems that use such interfaces.

Changes:

• Moved the ifa->ifa_addr NULL check out of the loop-continuation condition and into the loop body in both the counting and populating loops, so NULL entries are skipped instead of stopping enumeration.
• Kept the counting loop and populating loop guards identical, ensuring the reserved capacity matches the number of emitted interfaces.
• Removed a trailing blank line at the end of the file.