Implement gzfwrite and add null buffer check to gzfread

[brian-pane]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 4.54545% with 21 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libz-rs-sys/src/gz.rs	4.54%	21 Missing ⚠️

Flag	Coverage Δ
fuzz-compress	?
fuzz-decompress	?
test-aarch64-apple-darwin	87.74% <4.54%> (-0.02%)	⬇️
test-x86_64-apple-darwin	86.58% <4.54%> (-0.66%)	⬇️
test-x86_64-unknown-linux-gnu	88.51% <0.00%> (-0.20%)	⬇️

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Files with missing lines	Coverage Δ
libz-rs-sys/src/gz.rs	91.26% <4.54%> (-1.12%)	⬇️

... and 4 files with indirect coverage changes

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