Prevent crash on invalid server-ipv6 argument

`get_addr_generic()` expects `openvpn_getaddrinfo()` to return a newly
allocated struct, but getaddrinfo(3) failure leaves `*ai = NULL` as-is.

On OpenBSD, unlike free(3), freegetaddrinfo(3) requires a valid struct,
thus callers must check the argument to avoid NULL-deref or double-free:

```
$ openvpn --server-ipv6 ''
2025-12-06 11:59:18 RESOLVE: Cannot resolve host address: :[AF_INET6] (no address associated with name)
Segmentation fault (core dumped)
```

Guard against empty `ai`, i.e. failure, like similar code already does:

```
$ ./openvpn --server-ipv6 ''
2025-12-06 12:05:11 RESOLVE: Cannot resolve host address: :[AF_INET6] (no address associated with name)
Options error: error parsing --server-ipv6 parameter
Use --help for more information.
```

Spotted through a configuration typo "server-ipv6 fd00:/64" with 2.6.17,
reproduced with and tested against 2.7rc3 on OpenBSD/amd64 7.8-current.

NB: Standards are unclear wrt. freeaddrinfo(3)'s NULL handling;
    Linux, FreeBSD and illumos do check it and thus not crash.

Github: fixes OpenVPN/openvpn#930

Change-Id: I99a6604fdfc682f9609bfe7672aa78285084dcb9
Signed-off-by: Klemens Nanni <kn@openbsd.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1418
Message-Id: <20251207210529.9949-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34870.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 0ff66c056f)

src/openvpn/socket.c

@@ -170,7 +170,10 @@ get_addr_generic(sa_family_t af, unsigned int flags, const char *hostname,
         *sep = '/';
     }
 out:
-    freeaddrinfo(ai);
+    if (ai)
+    {
+        freeaddrinfo(ai);
+    }
     free(var_host);
 
     return ret;