Unsound issue in Trailer

[CXWorks]

Hi, thanks for your time to read this issue.

Our static analyzer find a potential unsound issue in the construction of Trailer, where it doesn't provide enough check to ensure the soundness.

trailer/src/lib.rs

Lines 18 to 25 in d474984

	pub fn new(capacity: usize) -> Trailer<T> {
	unsafe {
	let trailer = Trailer::allocate(capacity);
	let ptr = trailer.ptr as *mut T;
	ptr.write(T::default());
	trailer
	}
	}

The constructor does check the T is not a ZST in rust, and allocating with size 0 is considered as undefined behaviors in Rust. A poc code like below can work:

use trailer::Trailer;
#[derive(Default)]
struct Zst;

fn main() {
    let mut a = Trailer::<Zst>::new(0);
    drop(a);
}

Thanks again for your time.