This PR fixes this error when compiling a release build:

<img width="504" alt="Screenshot 2024-01-30 at 2 30 38 PM"
src="https://github.com/zed-industries/zed/assets/1486634/96470735-2b9e-4945-b4c3-c86ef0168b8c">

`DebugBelow` only exists when compiling with `debug_assertions`, so we
only want to implement it using that same criterion.

Release Notes:

- N/A