Vendor in c/image with sigstore support

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/google.golang.org/grpc/service_config.go

@@ -218,7 +218,7 @@ type jsonSC struct {
 }
 
 func init() {
-	internal.ParseServiceConfigForTesting = parseServiceConfig
+	internal.ParseServiceConfig = parseServiceConfig
 }
 func parseServiceConfig(js string) *serviceconfig.ParseResult {
 	if len(js) == 0 {
@@ -381,6 +381,9 @@ func init() {
 //
 // If any of them is NOT *ServiceConfig, return false.
 func equalServiceConfig(a, b serviceconfig.Config) bool {
+	if a == nil && b == nil {
+		return true
+	}
 	aa, ok := a.(*ServiceConfig)
 	if !ok {
 		return false