fix unsound return lifetime on Weak*Mut::as_mut_slice()

[Manishearth]

This returns a mutable borrow while relinquishing mutable access to the source type. This is prone to lifetime extension bugs, but it's internal so it doesn't actually matter.

Worth fixing anyway.

(This issue was discovered by an audit performed using Gemini)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
fuzz-compress	?
fuzz-decompress	?
test-aarch64-apple-darwin	90.00% <100.00%> (+<0.01%)	⬆️
test-aarch64-unknown-linux-gnu	85.42% <100.00%> (-0.01%)	⬇️
test-i686-unknown-linux-gnu	85.02% <100.00%> (-0.01%)	⬇️
test-x86_64-apple-darwin	88.28% <100.00%> (+<0.01%)	⬆️
test-x86_64-unknown-linux-gnu	90.81% <100.00%> (-1.23%)	⬇️

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

Files with missing lines	Coverage Δ
zlib-rs/src/deflate/hash_calc.rs	100.00% <100.00%> (ø)
zlib-rs/src/weak_slice.rs	100.00% <100.00%> (ø)

... and 6 files with indirect coverage changes

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[folkertdev]

Nice, thanks!