FlatArray<T> will SEGFAULT when referencing a Toasted array

[mhov]

In v0.18.0, when using the new FlatArray type for zero-copy arrays, if the array is Toasted data, even just referencing it in a parameter will cause a segfault. I have a failing test for replication here https://github.com/mhov/pgx/tree/flat_arr_toast_fail

cargo test --features pg18,pg_test --no-default-features borrow_test_arr_data_ptr_toasted

    #[pg_extern]
    fn borrow_get_arr_nelems(arr: &FlatArray<'_, i32>) -> libc::c_int {
        arr.nelems() as _
    }

    #[pg_test]
    fn borrow_test_arr_data_ptr_toasted() {
        // 2000 i32s should be the TOAST threshold.
        Spi::run("CREATE TEMP TABLE test_array (arr integer[])")
            .expect("failed to create temp table");
        Spi::run("INSERT INTO test_array SELECT array_agg(i) FROM generate_series(1, 2500) i")
            .expect("failed to insert into temp table");
        let len = Spi::get_one::<i32>("SELECT borrow_get_arr_nelems(arr) FROM test_array");
        assert_eq!(len, Ok(Some(2500)));
    }

This will fix it, but I'm 0% confident in the soundness of de-toasting here. As always I'd defer to @workingjubilee

unsafe impl<T: ?Sized> BorrowDatum for FlatArray<'_, T> {
    const PASS: layout::PassBy = layout::PassBy::Ref;
    unsafe fn point_from(ptr: ptr::NonNull<u8>) -> ptr::NonNull<Self> {
        unsafe {
+            let toast = ptr::NonNull::new(pg_sys::pg_detoast_datum(ptr.as_ptr().cast())).unwrap();

            let len =
-                varlena::varsize_any(ptr.as_ptr().cast()) - mem::size_of::<pg_sys::ArrayType>();
+                varlena::varsize_any(toast.as_ptr().cast()) - mem::size_of::<pg_sys::ArrayType>();
            ptr::NonNull::new_unchecked(
-                ptr::slice_from_raw_parts_mut(ptr.as_ptr(), len) as *mut Self
+                ptr::slice_from_raw_parts_mut(toast.as_ptr(), len) as *mut Self
            )
        }
    }
}