fix(gix-bitmap): Reject chunk length that overflows 32-bit usize

[Mrmaxmeier]

Hi,
I ran into this panic with the ewah fuzz target on 32-bit platforms (i686, WASM):

panicked at /rustc/.../library/alloc/src/raw_vec/mod.rs:28:5: capacity overflow

  19: core::panicking::panic_fmt
             at /rustc/.../library/core/src/panicking.rs:80:14
  20: alloc::raw_vec::capacity_overflow
             at /rustc/.../library/alloc/src/raw_vec/mod.rs:28:5
  25: <alloc::vec::Vec<u64>>::with_capacity
             at /rustc/.../library/alloc/src/vec/mod.rs:525:9
  26: gix_bitmap::ewah::decode
             at /tmp/gitoxide/gix-bitmap/src/ewah.rs:22:19

It happens in gix_bitmap::ewah::decode(&[0, 0, 0, 0, 0x20, 0, 0, 0]) because len * 8 overflows usize.
This PR uses a checked multiplication instead. saturating_mul would also work but the explicit error message might be nice to have. Let me know if I should replace the patch with a single-line saturating_mul one and/or add a test.

Thanks!

[Sebastian Thiel (Byron)]

Thanks so much!