Fix export of provided EC keys

The exporter freed a buffer too soon, and there were attempts to use
its data later, which was overwritten by something else at that
point.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11358)

providers/implementations/keymgmt/ec_kmgmt.c

@@ -352,13 +352,10 @@ int ec_export(void *keydata, int selection, OSSL_CALLBACK *param_cb,
     if ((selection & OSSL_KEYMGMT_SELECT_OTHER_PARAMETERS) != 0)
         ok = ok && otherparams_to_params(ec, tmpl, NULL);
 
-    if (!ok
-        || (params = OSSL_PARAM_BLD_to_param(tmpl)) == NULL)
-        goto err;
-
+    if (ok && (params = OSSL_PARAM_BLD_to_param(tmpl)) != NULL)
         ok = param_cb(params, cbarg);
+
     OSSL_PARAM_BLD_free_params(params);
-err:
     OSSL_PARAM_BLD_free(tmpl);
     OPENSSL_free(pub_key);
     return ok;