[yuxi-liu-wired]

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Problem

Two integer overflows in the legacy-zip implode decoder (src/legacy/). Both panic in debug
builds and wrap in release builds, where the read then fails with an error. The PR template asks
for all methods to stay panic-free.

1. read_huffman_code (implode.rs) computes the number of code-length bytes as
   (byte + 1) as usize with byte: u8. A header byte of 0xff means 256 bytes, which is valid
   (the literal table has 256 symbols), and 0xff + 1 overflows u8:

   panicked at src/legacy/implode.rs:29:21: attempt to add with overflow
   

   hwzip, which this decoder is ported from, widens first: num_bytes = (size_t)byte + 1.

2. HuffmanDecoder::huffman_decode (huffman.rs) computes the bits left as
   length - is.position_in_bits(). The bit reader can already be past the end of the data
   when a symbol is decoded, so the subtraction underflows:

   panicked at src/legacy/huffman.rs:141:67: attempt to subtract with overflow
   

   Reached through ZipArchive and through the streaming reader
   (read_zipfile_from_stream) on a truncated or crafted imploded entry.

Change

• num_bytes = usize::from(byte) + 1.
• In huffman_decode, compute the remaining bits with saturating_sub; when none are left,
  return an UnexpectedEof error (the stream ended in the middle of a symbol). The second
  lookup is clamped the same way.

Test

In src/legacy/implode.rs, next to the existing Hamlet test:

• code_length_table_with_256_bytes: a 256-entry code-length table (header byte 0xff) must
  decode. Panics on master.
• truncated_stream_is_an_error: the Hamlet implode stream truncated at every length must
  return an error or a prefix of the expected output, never panic. Panics on master at 93 of 249
  bytes.

The existing legacy tests pass unchanged. The suite passes on 1.88, stable and nightly with
default features, --no-default-features, --all-features, and the two single-deflate feature
sets; cargo clippy --all-targets -- -D warnings (default, --no-default-features,
--features legacy-zip, --all-features) and cargo doc --no-deps are clean; cargo fmt --check --all is clean.

Found with a fuzzer that reads generated archives through the streaming reader.