BUG/MINOR: ssl: Stop leaking err in ssl_sock_load_ocsp()

Previously performing a config check of `.github/h2spec.config` would report a
20 byte leak as reported in GitHub Issue #2082.

The leak was introduced in a6c0a59e9af65180c3ff591b91855bea8d19b352, which is
dev only. No backport needed.

src/ssl_sock.c

@@ -1296,6 +1296,8 @@ out:
 	if (warn)
 		free(warn);
 
+	free(err);
+
 	return ret;
 }