emqx/kuiper: xsql/ast.go; 3 LoC

Found a possible issue in [emqx/kuiper](https://www.github.com/emqx/kuiper) at [xsql/ast.go](https://github.com/emqx/kuiper/blob/91eacc9baea40bd1dedf0267c18ef39a3296c829/xsql/ast.go#L410-L412)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first issue it finds, so please do not limit your consideration to the contents of the below message.

>

[Click here to see the code in its original context.](https://github.com/emqx/kuiper/blob/91eacc9baea40bd1dedf0267c18ef39a3296c829/xsql/ast.go#L410-L412)

Click here to show the 3 line(s) of Go which triggered the analyzer.

```go
for _, c := range n {
Walk(v, &c)
}

```

Click here to show extra information the analyzer produced.

```
No path was found through the callgraph that could lead to a function which writes a pointer argument.

No path was found through the callgraph that could lead to a function which passes a pointer to third-party code.

root signature {Walk 2} was not found in the callgraph; reference was passed directly to third-party code
```

Leave a reaction on this issue to contribute to the project by classifying this instance as a **Bug** :-1:, **Mitigated** :+1:, or **Desirable Behavior** :rocket:
See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: 91eacc9baea40bd1dedf0267c18ef39a3296c829

Inspect xsql/ast.go around lines 410-412, then trace the Walk call and the range-loop variable c. Determine whether the analyzer finding represents a bug, mitigated behavior, or desirable behavior, and leave the corresponding reaction-based classification on the issue.

Written by the indexing model from the issue text.