bug: CFG building panics in debug builds

This happens when testing against Preact's codebase.
<img width="1512" alt="image" src="https://github.com/oxc-project/oxlint-ecosystem-ci/assets/22823424/1515dfec-9ac6-4aa2-a6d7-c3b826a525d1">

We should consider running tests with debug assertions enabled.