http-push: prevent format overflow warning with gcc >= 9

In function 'finish_request', inlined from 'process_response' at http-push.c:248:2: http-push.c:587:4: warning: '%s' directive argument is null [-Wformat-overflow=] 587 | fprintf(stderr, "Unable to get pack file %s\n%s", | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 588 | request->url, curl_errorstr); | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ request->url is needed for the error message if there was a failure during fetch but was being cleared unnecessarily earlier. note that the leak is prevented by calling release_request unconditionally at the end. Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com> Suggested-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>