parser: unsafe block should expect `{` as well

[liigo]

fn main() {
    unsafe // {
        std::mem::transmute::<f32, u32>(1.0);
    // }
}

error: expected one of `extern` or `fn`, found `std`
 --> <anon>:3:9
  |
3 |         std::mem::transmute::<f32, u32>(1.0);
  |         ^^^

error: aborting due to previous error

[eddyb]

This is because it's not parsing an unsafe block but a nested function.
A bit weird, not sure there's a nice way to reconcile them.