libcrux has All-Zero Key Generation Upon Catastrophic RNG Failure

Summary

The libcrux-ed25519 key generation samples Ed25519 secret keys from a provided CSPRNG in a loop for up to 100 attempts until a non-zero key is found. If a non-zero key could not be sampled within 100 attempts the key generation function would silently continue with an all-zero buffer as the secret key. ## Impact This bug only occurs in the event of a catastrophic failure of the CSPRNG, but would allow anyone to forge signatures under the resulting static signing key. ## Mitigation Instead of silently continuing with an all-zero signing key, starting from version `0.0.7` key generation will error in the case of 100 failed attempts at sampling a valid key.