ntru-rs has unsound FFI: Wrong API usage causes write past allocated area

The following usage causes undefined behavior.

let kp: ntru::types::KeyPair = …;
kp.get_public().export(Default::default())

When compiled with debug assertions, the code above will trigger a attempt to subtract with overflow panic before UB occurs. Other mistakes (e.g. using EncParams from a different key) may always trigger UB.

Likely, older versions of this crate are also affected, but have not been tested.