use-after-free in open_socket_out

[Carlos Carvalho]

Since 3.0.8 (the same in 3.1) we have this around line 300 in socket.c:

        break;
    }
    freeaddrinfo(res0);

    if (s < 0) {
        char buf[2048];
        for (res = res0, j = 0; res; res = res->ai_next, j++) {
            if (errnos[j] == 0)
                continue;
            if (inet_ntop(res->ai_family, res->ai_addr->sa_data + 2, buf, sizeof buf) == NULL)
                strlcpy(buf, "*inet_ntop failed*", sizeof buf);
            rsyserr(FERROR, errnos[j], "failed to connect to %s (%s)", h, buf);
        }
        s = -1;
    }

res0 is freed but keeps its value so there's access to freed memory in
res->ai* in the call of inet_ntop if s < 0. This might explain why
we've got a few cases of inet_top failed in the logs. I suggest the
attached obvious patch.

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