Empty input panics in three raw unpadding implementations

{
  "title": "Empty input panics in three raw unpadding implementations",
  "body": "## Description\n\nIn block-padding 0.4.2, `Pkcs7::raw_unpad`, `Iso10126::raw_unpad`, and `AnsiX923::raw_unpad` panic on an empty slice. `Padding::raw_unpad` says malformed padding returns an error, and an empty slice has no final count byte.\n\n## Steps to Reproduce\n\n```rust\nuse block_padding::{Padding, Pkcs7};\n\nassert!(Pkcs7::raw_unpad(&[]).is_err());\n```\n\n## Expected Behavior\n\nThe call returns `Err` for the malformed block.\n\n## Actual Behavior\n\nThe call panics instead of returning a result.\n\n## Environment\n\nblock-padding 0.4.2; reproduced in debug and release builds.",
  "validity": "reported",
  "upstream_status": "not_reported"
}