fix logic error in blocksort

[folkertdev]

Fixes #137

The analysis in the issue is correct I think, the loop was doing something slightly subtle. I used alive2 to verify that the version in this PR is equivalent, but it is more efficient (based on the assembly and it not looping).

• https://godbolt.org/z/s8d3xn3q9
• https://alive2.llvm.org/ce/z/mNheCz

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libbz2-rs-sys/src/blocksort.rs	87.50%	1 Missing ⚠️

Flag	Coverage Δ
fuzz-compress_then_decompress	50.68% <0.00%> (-0.05%)	⬇️
fuzz-decompress_chunked	30.20% <0.00%> (-1.39%)	⬇️
test-aarch64-apple-darwin	90.02% <90.90%> (-0.01%)	⬇️
test-aarch64-unknown-linux-gnu	89.87% <90.90%> (+<0.01%)	⬆️
test-i686-unknown-linux-gnu	89.87% <90.90%> (+<0.01%)	⬆️
test-x86_64-apple-darwin	89.87% <90.90%> (+<0.01%)	⬆️
test-x86_64-unknown-linux-gnu	90.02% <90.90%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
test-libbz2-rs-sys/src/lib.rs	95.90% <100.00%> (+0.01%)	⬆️
libbz2-rs-sys/src/blocksort.rs	98.96% <87.50%> (-0.17%)	⬇️

... and 2 files with indirect coverage changes

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