returning_works encounters UB, will fail in Rust 1.83

I am filing this issue because this project's tests failed in a crater run for this PR: https://github.com/rust-lang/rust/pull/130251 which should land in Rust 1.83.

The problem is that this crate can (under some conditions I do not understand but are encountered by returning_works) apply a negative offset to a null pointer via ptr::offset, which would cause the pointer to wrap around the address space. The standard library is getting a debug assertion for such wrapping offsets. I'm pretty sure that this is just a missing check if the pointer is null in the Drop impl for Mapping. Nothing too wild happens currently because this just tries to munmap an address range that isn't mapped and that is well-defined. But we already tell LLVM that ptr::offset does not wrap, and in this test it does, so surprise optimizations may ensue.

In any case, Miri can also be used to detect this bug today:

╰ ➤ RUSTFLAGS=-Awarnings cargo +nightly miri test returning_works
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.00s
     Running unittests src/lib.rs (target/miri/x86_64-unknown-linux-gnu/debug/deps/apu_pcengines_hal-b14fa427df7385eb)

running 1 test
test tests::returning_works ... error: Undefined Behavior: out-of-bounds pointer arithmetic: expected a pointer to the end of 5376 bytes of memory, but got a null pointer
   --> src/lib.rs:213:17
    |
213 |                 self.gpio.offset((FCH_GPIO_OFFSET as isize) * -1),
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer arithmetic: expected a pointer to the end of 5376 bytes of memory, but got a null pointer