getifaddrs: fix UB via taking address of null pointer union dereference

getifaddrs: fix UB via taking address of null pointer union dereference

getifaddrs computes &ctx->first->ifa even if ctx->first is NULL. While
this shouldn't be possible on the success path because the loopback
interface is hardcoded into the kernel, this is still possible on the
error path (for example, if __rtnetlink_enumerate couldn't create a
socket due to exceeding the fd limit).