Calling debug_assert! with two arguments causes ICE

The following code results in a panic:

```
//
//
//
//
//
// No error if the file is too short
//
//
//
//
// line 11
fn main() {
    debug_assert!( "me", "me" );
}
```

producing

```
task 'rustc' panicked at 'index out of bounds: the len is 12 but the index is 12', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libsyntax/lib.rs:1
```
