Simulation crashes when compiled without NDEBUG

Removing `-DNDEBUG=1` from `CFLAGS` results in failed simulation tests.

Oddly, I don't actually much in our code that depends on `NDEBUG`, and we use our own `ASSERT()` macro rather than `assert()`. But the crashes are still there regardless. ‾\\\_(ツ)\_/‾

Reproduce the simulation test failures after removing -DNDEBUG=1 from CFLAGS, then compare the failing behavior with and without NDEBUG. Start by tracing the crash through the simulation tests and the project's ASSERT() macro. Done means the simulation tests pass when compiled without NDEBUG.

Written by the indexing model from the issue text.