Prevent container.Kill() from crashing if container.cmd is nil

Upstream-commit: b32436cd2ea1f206cf5758170172f46dafb1f43c
Component: engine

components/engine/container.go

@@ -356,6 +356,9 @@ func (container *Container) monitor() {
 }
 
 func (container *Container) kill() error {
+	if container.cmd == nil {
+		return nil
+	}
 	if err := container.cmd.Process.Kill(); err != nil {
 		return err
 	}