Socket leak in `neu_conn_eth_check_interface()` on `ioctl()` failure

[K-ANOY]

There is a socket fd leak in neu_conn_eth_check_interface() when the interface hardware-address lookup fails.

File: src/connection/connection_eth.c

Function: neu_conn_eth_check_interface

Relevant code:

int neu_conn_eth_check_interface(const char *interface)
{
    struct ifreq ifr = { 0 };
    int          fd  = socket(PF_PACKET, SOCK_RAW, ETH_P_IPV6);
    int          ret = -1;

    if (fd <= 0) {
        return -1;
    }

    snprintf(ifr.ifr_name, IFNAMSIZ, "%s", interface);
    ret = ioctl(fd, SIOCGIFHWADDR, &ifr);
    if (ret != 0) {
        return -1;
    }

    close(fd);
    return ret;
}

The success path closes fd before returning:

close(fd);
return ret;

But if ioctl(fd, SIOCGIFHWADDR, &ifr) fails, the function returns -1
without closing the socket. This leaks one socket fd per failed interface check.

Suggested fix: close fd before returning from the ioctl() error path.

Also, socket() can legally return fd 0, so the check should probably be
fd < 0 instead of fd <= 0.

[fengzeroz]

#2907