archive: fix check for missing url

Running "git archive --remote" checks that we have at least one url for the remote. It does so by looking at remote.url[0], but that won't work; if we have no url at all, then remote.url will be NULL, and we'll segfault. Check url_nr instead, which is a more direct way of asking what we want. You can trigger the segfault like this: git -c remote.foo.vcs=bar archive --remote=foo but I didn't bother adding a test. This is the tip of the iceberg for no-url remotes, and a later patch will improve that situation. I just wanted to clean up this bug so it didn't make further refactoring of this code more confusing. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>