Reject a nil config in New instead of panicking

New dereferenced config.RepoMap without checking the config pointer, so
New(nil) panicked. NewConfig returns a nil config alongside its error, so a
caller that ignores the error reaches this path.

Also move client construction after validation, so the invalid-input paths
no longer allocate a client they immediately discard.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

Author: rdimitrov

metadata/multirepo/multirepo.go

@@ -122,10 +122,8 @@ func NewConfig(repoMap []byte, roots map[string][]byte) (*MultiRepoConfig, error
 
 // New returns a multi-repository TUF client. All repositories described in the provided map file are initialized too
 func New(config *MultiRepoConfig) (*MultiRepoClient, error) {
-	// create a multi repo client instance
-	client := &MultiRepoClient{
-		Config:     config,
-		TUFClients: map[string]*updater.Updater{},
+	if config == nil {
+		return nil, fmt.Errorf("no multi-repository config provided")
 	}
 
 	// validate the map file before initializing anything, so that a malformed map
@@ -135,6 +133,12 @@ func New(config *MultiRepoConfig) (*MultiRepoClient, error) {
 		return nil, err
 	}
 
+	// create a multi repo client instance
+	client := &MultiRepoClient{
+		Config:     config,
+		TUFClients: map[string]*updater.Updater{},
+	}
+
 	// create TUF clients for each repository listed in the map file
 	if err := client.initTUFClients(); err != nil {
 		return nil, err

metadata/multirepo/multirepo_test.go

@@ -228,3 +228,12 @@ func TestNewRejectsConfigWithoutRepoMap(t *testing.T) {
 		t.Fatal("New() should reject a config with no repository map")
 	}
 }
+
+func TestNewRejectsNilConfig(t *testing.T) {
+	// NewConfig returns a nil config alongside its error, so a caller that
+	// ignores the error passes nil straight into New.
+	_, err := New(nil)
+	if err == nil {
+		t.Fatal("New() should reject a nil config")
+	}
+}