Fix a potential panic

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/store/sandbox/sandbox.go

@@ -37,7 +37,7 @@ type Sandbox struct {
 	// CNI network namespace client
 	NetNS *NetNS
 	// StopCh is used to propagate the stop information of the sandbox.
-	store.StopCh
+	*store.StopCh
 }
 
 // NewSandbox creates an internally used sandbox type. This functions reminds
@@ -46,7 +46,7 @@ func NewSandbox(metadata Metadata, status Status) Sandbox {
 	s := Sandbox{
 		Metadata: metadata,
 		Status:   StoreStatus(status),
-		StopCh:   store.StopCh(make(chan struct{})),
+		StopCh:   store.NewStopCh(),
 	}
 	if status.State == StateNotReady {
 		s.Stop()