deduplicate some FSE functions and make them (more) safe

[folkertdev]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/compress/zstd_compress_superblock.rs	0.00%	7 Missing ⚠️
lib/compress/zstd_compress.rs	75.00%	4 Missing ⚠️
lib/compress/zstd_compress_sequences.rs	83.33%	4 Missing ⚠️
lib/compress/zstd_opt.rs	0.00%	3 Missing ⚠️
lib/compress/hist.rs	66.66%	2 Missing ⚠️

Flag	Coverage Δ
test-aarch64-apple-darwin	33.74% <80.00%> (+0.01%)	⬆️
test-aarch64-unknown-linux-gnu	32.56% <80.00%> (-0.05%)	⬇️
test-i686-unknown-linux-gnu	32.65% <80.00%> (-0.05%)	⬇️
test-x86_64-unknown-linux-gnu	33.76% <80.00%> (+0.03%)	⬆️

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

Files with missing lines	Coverage Δ
lib/common/fse.rs	53.38% <100.00%> (+6.78%)	⬆️
lib/compress/fse_compress.rs	74.87% <100.00%> (-1.11%)	⬇️
lib/compress/huf_compress.rs	81.82% <100.00%> (ø)
lib/compress/hist.rs	71.62% <66.66%> (ø)
lib/compress/zstd_opt.rs	0.00% <0.00%> (ø)
lib/compress/zstd_compress.rs	36.79% <75.00%> (+0.62%)	⬆️
lib/compress/zstd_compress_sequences.rs	69.10% <83.33%> (+0.07%)	⬆️
lib/compress/zstd_compress_superblock.rs	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

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