Trying to generate a musig2 partial signature with a secret nonce that was generated with a public key that does not match the
siging key's public key will trigger secp256k1's illegal callback (which calls abort()) and crash the application.

=> Here we verify that the secret nonce matches the siging keys before we call secp256k1_musig_partial_sign().
The verification method is a bit hackish (we extract the public key from the secret nonce blob) because secp256k1 does not export the methods we need
to do this cleanly.