Soundness Bug in this crate

Hi there!

We scanned the most popular libraries on crates.io and found some memory safety bugs in this library.

## ::free_part

### PoC
```rust
// src/main.rs
use dlmalloc::{Allocator, Dlmalloc};

fn main() {
let d = Dlmalloc::new();
let sys = d.allocator();

// - ptr = null => violates ptr::add precondition in the mremap-failure fallback.
// - oldsize < newsize => causes usize underflow in `oldsize - newsize` in release.
let ptr = core::ptr::null_mut::();
let oldsize = 0usize;
let newsize = 1usize;

// This is a safe call (trait method is safe) that reaches internal unsafe code.
let _ = Allocator::free_part(sys, ptr, oldsize, newsize);
}
```

### Miri Output
```text
error: Undefined Behavior: in-bounds pointer arithmetic failed: attempting to offset pointer by 1 byte, but got null pointer
--> /home/ccuu/Desktop/llm-detector/experiments/cache/crates_src/dlmalloc/0.2.12/dlmalloc-0.2.12/src/unix.rs:60:26
|
60 | libc::munmap(ptr.add(newsize).cast(), oldsize - newsize) == 0
| ^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
|
= 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:
= note: inside `::free_part` at /home/ccuu/Desktop/llm-detector/experiments/cache/crates_src/dlmalloc/0.2.12/dlmalloc-0.2.12/src/unix.rs:60:26: 60:42
note: inside `main`
--> src/main.rs:22:13
|
22 | let _ = Allocator::free_part(sys, ptr, oldsize, newsize);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error
```

We appreciate your work on this crate and hope this report helps improve its safety.