When reading back an array from Futhark to Rust using values, if the output buffer has more capacity than the array's length, it will crash.

Here's a concrete example where I allocate a buffer of 5 elements for an array of 2.
Futhark code:

entry make_array_of_len (n: i64) : []i64 =
  iota n

Rust code:

use futhark_lib::{Config, Context, backends};
type Backend = backends::C;

fn main() {
    let config = Config::<Backend>::new();
    let context = Context::new(config);
    
    let mut buffer = Vec::with_capacity(5);
    let array = context.entry_make_array_of_len(2).unwrap();
    array.values(&mut buffer);
    
    dbg!(&buffer);
}

And when executed:

thread 'main' panicked at /.../futhark_lib.rs:316:21:
attempt to subtract with overflow

The issue was the code generated at line 79 of "template/array.rs":

out.reserve(len - out.capacity());

Specifically len - out.capacity() would underflow if out's capacity was already greater than len.

So I added a check to make sure it reserves more memory only when it is needed.
I tried the patch on the code earlier and it did fix the issue:

     Running `target/debug/fut_cargo`
[src/main.rs:11:5] &buffer = [
    0,
    1,
]