Fix segfault in error handling of gh repo rename

[satoqz]

Hi, the updateRemote() function used in the gh repo rename command does not always guarantee the *ghcontext.Remote return value to be != nil as seen below:

cli/pkg/cmd/repo/rename/rename.go

Lines 146 to 168 in ec812a1

	func updateRemote(repo ghrepo.Interface, renamed ghrepo.Interface, opts *RenameOptions) (*ghContext.Remote, error) {
	cfg, err := opts.Config()
	if err != nil {
	return nil, err
	}
	protocol := cfg.GitProtocol(repo.RepoHost())
	remotes, err := opts.Remotes()
	if err != nil {
	return nil, err
	}
	remote, err := remotes.FindByRepo(repo.RepoOwner(), repo.RepoName())
	if err != nil {
	return nil, err
	}
	remoteURL := ghrepo.FormatRemoteURL(renamed, protocol)
	err = opts.GitClient.UpdateRemoteURL(context.Background(), remote.Name, remoteURL)
	return remote, err
	}

However the caller of this function assumes this to be the case and always de-references remote.Name even if the function returns an error:

cli/pkg/cmd/repo/rename/rename.go

Lines 136 to 141 in ec812a1

	remote, err := updateRemote(currRepo, newRepo, opts)
	if err != nil {
	fmt.Fprintf(opts.IO.ErrOut, "%s Warning: unable to update remote %q: %v\n", cs.WarningIcon(), remote.Name, err)
	} else if opts.IO.IsStdoutTTY() {
	fmt.Fprintf(opts.IO.Out, "%s Updated the %q remote\n", cs.SuccessIcon(), remote.Name)
	}

... which then leads to a segfault.

In my case, this edge case was triggered by renaming a repository that had previously been moved between organizations. My local remote was still outdated, which causes remotes.FindByRepo() to return nil.

This fix adds a new error message that handles this case which looks like below:

! Warning: unable to update remote: no matching remote found

[cliAutomation]

Hi! Thanks for the pull request. Please ensure that this change is linked to an issue by mentioning an issue number in the description of the pull request. If this pull request would close the issue, please put the word 'Fixes' before the issue number somewhere in the pull request body. If this is a tiny change like fixing a typo, feel free to ignore this message.

[williammartin]

LGTM thanks!