Fix segfault in list pattern match

[rtfeldman]

Claude fixed this (Zed Agent) with one prompt:

in the new Zig compiler, this works:
```
module [last]

last = |l|
    match l {
        [.., e] => Ok(e)
        [] => Err(EmptyList)
    }
```

this works too:

```
module [last]

last = |l|
    match l {
        _ => Err(EmptyList)
        [.., e] => Ok(e)
    }
```

however, this segfaults:

```
module [last]

last = |l|
    match l {
        [] => Err(EmptyList)
        [.., e] => Ok(e)
    }
```

please reproduce in a `zig build test` test somewhere (none of the 3 should segfault!) and then fix the segfault. please add it to an existing test file; don't create a new one for this.