Rustdoc fails in Option::get_ref if there's a module with no public items

[sfackler]

If I have something like

#[link(name="thing")];

pub mod foo {
}

running rustdoc fails:

~ ❯ rustdoc test.rs
task '<unnamed>' failed at 'called `Option::get_ref()` on a `None` value', /build/rust-git/src/rust/src/libstd/option.rs:247

but adding a public struct to foo fixes it:

#[link(name="thing")];

pub mod foo {
    pub struct Baz;
}