Fix nil pointer deref in dump-snapshot --redacted

container.PullOptions is nil if the service is deployed without
`--with-registry-auth`.  Observed on client+server 19.03.2.

Signed-off-by: Trapier Marshall <trapier.marshall@docker.com>

cmd/swarm-rafttool/dump.go

@@ -200,15 +200,19 @@ func dumpSnapshot(swarmdir, unlockKey string, redact bool) error {
 			if task != nil {
 				if container := task.Spec.GetContainer(); container != nil {
 					container.Env = []string{"ENVVARS REDACTED"}
-					container.PullOptions.RegistryAuth = "REDACTED"
+					if container.PullOptions != nil {
+						container.PullOptions.RegistryAuth = "REDACTED"
+					}
 				}
 			}
 		}
 		for _, service := range s.Store.Services {
 			if service != nil {
 				if container := service.Spec.Task.GetContainer(); container != nil {
 					container.Env = []string{"ENVVARS REDACTED"}
-					container.PullOptions.RegistryAuth = "REDACTED"
+					if container.PullOptions != nil {
+						container.PullOptions.RegistryAuth = "REDACTED"
+					}
 				}
 			}
 		}