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

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>