Fix: do not use private openssl function to copy cert chain (#449)

Re: https://bugs.chromium.org/p/boringssl/issues/detail?id=558#c1

Since a recent change that puts boringssl's public API more in line with OpenSSL, there is a constness mismatch in the function type of `copy_X509` that may cause compilation failures.

As it turns out, the function we are using there (`sk_deep_copy`) is documented as a private API. We could use `sk_X509_deep_copy`, casting away the const in `copy_X509` to increment the refcount, but the existing function `X509_chain_up_ref` does exactly what we want here.

src/liblsquic/lsquic_enc_sess_ietf.c

@@ -2519,14 +2519,6 @@ iquic_esf_global_cleanup (void)
 }
 
 
-static void *
-copy_X509 (void *cert)
-{
-    X509_up_ref(cert);
-    return cert;
-}
-
-
 static struct stack_st_X509 *
 iquic_esf_get_server_cert_chain (enc_session_t *enc_session_p)
 {
@@ -2536,9 +2528,7 @@ iquic_esf_get_server_cert_chain (enc_session_t *enc_session_p)
     if (enc_sess->esi_ssl)
     {
         chain = SSL_get_peer_cert_chain(enc_sess->esi_ssl);
-        return (struct stack_st_X509 *)
-            sk_deep_copy((const _STACK *) chain, sk_X509_call_copy_func,
-                copy_X509, sk_X509_call_free_func, (void(*)(void*))X509_free);
+        return X509_chain_up_ref(chain);
     }
     else
         return NULL;