Internal command runner panics for some inputs

[sophiajt]

Recent parser changes panic for some seemingly correct inputs. Here's one example:

> ls | echo $it.name
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ShellError { error: MissingProperty { subpath: Source(Tagged { tag: Tag { origin: Some(00000000-0000-0000-0000-000000000000), span: Span { start: 11, end: 13 } }, item: "nothing" }), expr: Source(Tagged { tag: Tag { origin: Some(00000000-0000-0000-0000-000000000000), span: Span { start: 14, end: 18 } }, item: "name" }) }, cause: None }', src/libcore/result.rs:1165:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

[sophiajt]

Will be fixed by #677