UB: GlobalHeap should not be creatable from arbitrary pointers

[nathaniel-daniel]

Since GlobalHeap can be created from an arbitrary pointer, and code assumes heaps always have valid pointers, it is easy to cause UB:

use mimalloc_rust::heap::GlobalHeap;
use mimalloc_rust::heap::MiMallocHeap;
use mimalloc_rust::*;

#[global_allocator]
static GLOBAL_MIMALLOC: GlobalMiMalloc = GlobalMiMalloc;

fn main() {
    let new_heap = GlobalHeap {
        heap: std::ptr::null_mut(),
    };

    GlobalMiMalloc::replace_by(&MiMallocHeap::new(new_heap));
 }

[LemonHX]

you are right, I'll take a look