follow `MaybeUninit::slice_assume_init_ref` implementation

[folkertdev]

instead of a transmute. This is a bit safer if we ever end up changing the types (though that is unlikely)

see also https://doc.rust-lang.org/beta/src/core/mem/maybe_uninit.rs.html#979

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ
zlib-rs/src/deflate/pending.rs	97.01% <100.00%> (ø)

... and 2 files with indirect coverage changes