Compiling ambiguous macro invocation hangs forever

Trying to compile this code containing an ambiguous macro causes the compiler to hang forever and max out memory:

```
macro_rules! make_vec(
    (a $e1:expr $($(, a $e2:expr)*)*) => ([$e1 $($(, $e2)*)*]);
)

fn main() {
    let _ = make_vec!(a 1, a 2, a 3);
}
```

rustc 0.5
host: x86_64-apple-darwin
