Wouldn't this have been caught by an exhaustive unit test or even a fuzz test? I don't know what kind of testing practices are applied to projects like zfs, nor what kinds or amounts of tests functions like this are subject to, but I would imagine that especially for low-level functions like this, unit tests with all of the known edge cases would be written.

(yes this is very much an armchair opinion, I mostly do front-end JS for a living)