Add assert for the response of ffi negate interface
Add assert for the response of ffi negate interface

The interfaces for negate should always returns 1 as mentioned secp256k1.h L574, L563.
But in the future it might return 0 if the seckey or pubkey is invalid, but our type system doesn't allow that to ever happen.

apoelstra/rust-secp256k1,apoelstra/rust-secp256k1,apoelstra/rust-secp256k1,apoelstra/rust-secp256k1