[BUG] barrier cause double free

[luansh]

It run on 0.4.4

import "std/sync.zc"
import "std/thread.zc"
import "std/vec.zc"

def COUNT = 4;

fn worker(b: Barrier, delay: int) {
println "Before Sleep";
sleep_ms(delay);
b.wait();
println "After Sleep";
}

fn main() {
let ts = Vec::new();
let barrier = Barrier::new(COUNT);

for it in 0..COUNT {
ts.push(Thread::spawn(fn() { worker(barrier, it * 1000); }).unwrap());
}

for t in ts {
t.join();
}

sleep_ms(1000);
}

Terminal Output
Before Sleep
Before Sleep
Before Sleep
Before Sleep
After Sleep
After Sleep
After Sleep
double free or corruption (!prev)

It seems the last call to wait() cause the problem, and the last "After Sleep" can not be run