libcrux: Potential Panic on Overlong Ciphertext Buffer

Summary

An application that passes in a ciphertext buffer of length greater than `ptxt.len() + TAG_LEN` to `libcrux_chacha20poly1305::encrypt` or `libcrux_chacha20poly1305::xchacha20_poly1305::encrypt` would experience a panic. ## Impact An application where the length of the ciphertext buffer is under attacker control could be made to crash. ## Mitigation The fix makes it so that `libcrux_chacha20poly1305::encrypt` and `libcrux_chacha20poly1305::xchacha20_poly1305::encrypt` no longer panic in this case, but instead write out the ciphertext and tag into the first `ptxt.len() + TAG_LEN` bytes of the provided buffer.