fix panic in crc32 hash calc

[folkertdev]

fixes #219

The logic attempted to incorporate uninitialized bytes into the hash. That panics because of bounds checking, and we now handle it properly by having an upper bound to the slice index.

[codecov]

Codecov Report

Attention: Patch coverage is 76.19048% with 25 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
test-libz-rs-sys/src/deflate.rs	77.77%	22 Missing ⚠️
zlib-rs/src/deflate/hash_calc.rs	50.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
zlib-rs/src/deflate/hash_calc.rs	80.71% <50.00%> (-1.30%)	⬇️
test-libz-rs-sys/src/deflate.rs	98.16% <77.77%> (-1.06%)	⬇️