bslalg_rbtreeutil.t: Fix -Wunused warning.

Fixes:

```
../groups/bsl/bslalg/bslalg_rbtreeutil.t.cpp: In function ‘int main(int, char**)’:
../groups/bsl/bslalg/bslalg_rbtreeutil.t.cpp:5961: warning: unused variable ‘NODE’ [-Wunused-variable]
```

`NODE` is only referenced within a `BSLS_ASSERT`, so place the two statements in a `BSLS_ASSERT_IS_ACTIVE` check.