I feel like having "safety net" sections is a smell, too. Maybe it's due to limitations of the language? I'm not a C expert.

Safety nets make it seem like you're handling edge cases in a vaguely specified, ad-hoc way, which is prone to forgetting to add the safety net in at least some places, while the nets themselves are easy to mess up as well.

Could this code benefit from more typing perhaps? Automated checking of pre- and postconditions? Are there C (macro) libraries implement that in a usable way?