test edge cases in the public C api

[folkertdev]

we now cover all branches with dedicated tests.

[folkertdev]

miri reported UB here. I think the problem was that state got dropped and/or got confused about lifetimes here?

error: Undefined Behavior: constructing invalid value at .writer.buf: encountered a dangling reference (use-after-free)
    --> /home/folkertdev/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:734:14
     |
734  |     unsafe { intrinsics::typed_swap(x, y) }
     |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .writer.buf: encountered a dangling reference (use-after-free)
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
     = note: BACKTRACE on thread `coverage::ub`:
     = note: inside `std::mem::swap::<zlib_rs::inflate::State<'_>>` at /home/folkertdev/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:734:14: 734:42
note: inside `zlib_rs::inflate::end`
    --> /home/folkertdev/rust/zlib-rs/zlib-rs/src/inflate.rs:2226:5
     |
2226 |     core::mem::swap(&mut state, stream.state);
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `libz_rs_sys::inflateEnd`
    --> /home/folkertdev/rust/zlib-rs/libz-rs-sys/src/lib.rs:370:13
     |
370  |             zlib_rs::inflate::end(stream);

but I'm not 100% sure. This fix makes miri happy, and still passes all tests so I think we're good.