fixing err declaration

Author: jprobinson

auth/gcp/iam.go

@@ -40,6 +40,7 @@ type IAMClaimSet struct {
 // NewDefaultIAMVerifier will verify tokens that have the same default service account as
 // the server running this verifier.
 func NewDefaultIAMVerifier(ctx context.Context, cfg IAMConfig, clientFunc func(context.Context) *http.Client) (*auth.Verifier, error) {
+	var err error
 	eml := cfg.ServiceAccountEmail
 	if eml == "" {
 		eml, err = GetDefaultEmail(ctx, "", clientFunc(ctx))