Fix C API pointer provenance under Miri

[lopopolo]

Summary

Miri rejected several C API entry points in miniz_oxide_c_api as UB.

The bug pattern was the same in each case: take a raw pointer, turn it into a &u8 or &mut u8, and then widen that one-byte reference into a larger slice with slice::from_raw_parts or slice::from_raw_parts_mut.

That is not a sound way to construct the slice. This PR constructs the slices directly from the original raw pointers instead.

Changes

• add targeted #[test] witnesses for the failing Miri paths
• fix pointer-to-slice conversion in tdefl_compress
• fix the tdef callback bridge to pass out.as_ptr() instead of &(out[0])
• fix input/output slice construction in StreamOxide::try_new
• fix input slice construction in mz_adler32
• fix input slice construction in mz_crc32
• simplify zero-length slice handling in tdef

Testing

Ran:

cargo test -p miniz_oxide_c_api --lib miri_witness -- --nocapture
cargo +nightly miri test -p miniz_oxide_c_api --lib miri_witness -- --nocapture

Both pass.

Attribution

OpenAI Codex implemented the witnesses and fixes.

@lopopolo reviewed and validated the work.