new-counter-demo HTTP push panics when the target endpoint is unreachable

**What happened**:

The HTTP push method in `new-counter-demo/virtualprotocol/data/publish/http/client.go` dereferences `resp.Body` after `http.Post` returns an error. When the endpoint is unreachable, `resp` is nil and the mapper panics instead of returning from the failed publish attempt.

**What I expected to happen**:

A failed HTTP publish should be logged and returned without panicking the virtual protocol process.

**How to reproduce it (as minimally and precisely as possible)**:

1. Configure the HTTP push method with an unavailable endpoint.
2. Call the push method with one device data value.
3. Observe the nil pointer panic at `defer resp.Body.Close()`.

A focused regression test using `127.0.0.1:1` reproduces the failed request path without requiring a device or cluster.

**Anything else we need to know?**:

The fix is limited to the error path and preserves successful response handling.