attempting to tokenize lots of open parens results in a stack overflow

[frewsxcv]

https://gist.github.com/frewsxcv/3b427948da01685c95c26ab4a005c337

> cargo run
   Compiling proc-macro2-foo v0.1.0 (file:///private/tmp/proc-macro2-foo)
    Finished dev [unoptimized + debuginfo] target(s) in 2.4 secs
     Running `target/debug/proc-macro2-foo`

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
fish: 'cargo run' terminated by signal SIGABRT (Abort)

found via afl.rs

[dtolnay]

Looks like rustc has the same behavior as of 1.25.0-nightly.

fn main() {
    let s = ((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((...
}

thread 'rustc' has overflowed its stack
fatal runtime error: stack overflow
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[dtolnay]

Fixed by #231 + #232.