Checksum-only data can pass a requested version check

{
  "title": "Checksum-only data can pass a requested version check",
  "body": "## Description\n\nA requested version can be accepted when the decoded payload is empty. This affects `DecodeBuilder::with_check` and `DecodeBuilder::as_cb58`.\n\n## Steps to Reproduce\n\nWith bs58 0.5.1 and the `check` and `cb58` features enabled, run these assertions. Their success reproduces the discrepancy:\n\n```rust\nassert_eq!(bs58::decode(\"3QJmnh\").with_check(Some(93)).into_vec(), Ok(vec![]));\nassert_eq!(bs58::decode(\"45PJLL\").as_cb58(Some(120)).into_vec(), Ok(vec![]));\n```\n\n## Expected Behavior\n\nThe API promises to use the supplied version in verification. A requested version requires a nonempty payload whose first byte is that version, so these checksum-only inputs should be rejected.\n\n## Actual Behavior\n\nEach input contains only the four checksum bytes of an empty unversioned payload. When the payload is empty, the decoder compares the first checksum byte to the requested version and returns an empty payload if they match. Consequently, success with a requested version does not guarantee that the returned data contains that version; callers relying on that property can accept malformed versioned input.\n\n## Environment\n\nbs58 0.5.1 with `check` and `cb58`.\n\n## Additional Context\n\nSee [upstream issue #132](https://github.com/Nullus157/bs58-rs/issues/132).",
  "validity": "reported",
  "upstream_status": "not_reported"
}