httputil: work around NPE in signer.Add()

[glebsts]

If auth section is empty in config file, instance of Signer is initialized with default field values:
https://github.com/quay/clair/blob/v4.7.3/internal/httputil/signer.go#L29

Later e.g. in report command processing if configuration is used, method Add is used as "marks the authority in uri as one that expects signed requests":
https://github.com/quay/clair/blob/v4.7.3/cmd/clairctl/report.go#L145

In this method an entry is being added to use field of signer receiver no matter what. But if auth section was missing in config, use has default value of nil and produces assignment to entry in nil map.

This patch fixes internal logic, but I decided to keep URI parsing before actually checking for nil to give faster feedback to user if uri is e.g. malformed. Feel free to suggest change in such a flow.

Method Add is called from multiple commands, so I changed it inside the method, not pre-checking the calls.

Other Signer fields seem to be safe (ranging over nil map is ok).

There is no test file for signer, so I did not create a new test case I added one

[app-sre-bot]

Can one of the admins verify this patch?

[hdonnay]

Ah, good catch.

I'd prefer if the conditional were the other way, to keep it consistent with the other checks in that function. With that change, LGTM

[glebsts]

@hdonnay Hello, could you please clarify about "conditionals being other way"?
Do you mean smth like

if s.use == nil {
  return errors.New("authority map not initialized, perhaps missing auth section in config")
}

?