When containerd use this config:
```
[plugins."io.containerd.grpc.v1.cri".registry.mirrors]
[plugins."io.containerd.grpc.v1.cri".registry.mirrors."localhost:5000"]
endpoint = ["http://localhost:5000"]
```
Due to the `newTransport` function does not initialize the `TLSClientConfig` field.
Then use `TLSClientConfig` to cause nil pointer dereference
Signed-off-by: wanglei <wllenyj@linux.alibaba.com>
5f293d9ac4