improve usage of unsafe in `gix-pack` #3835

Job	Run time
test	4m 6s
test-32bit (armv7-linux-androideabi)	1m 9s
pure-rust-build	1m 55s
test-fast (windows-latest)	3m 51s
test-fast (macos-latest)	4m 7s
test-fast (ubuntu-latest)	3m 12s
cargo-deny (advisories)	28s
installation (win-msvc)	3m 59s
cargo-deny (bans licenses sources)	30s
installation (win-gnu)	3m 56s
installation (win32-msvc)	3m 57s
lint	3m 31s
installation (win32-gnu)	3m 56s
WebAssembly (wasm32-unknown-unknown)	55s
WebAssembly (wasm32-wasi)	37s
	40m 9s

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