Segmentation fault when using coro::Generator, coro::Task and CPUTreadpoolExecutor

Hi guys, I'm trying to use folly::coro in my project to get better performance, but this error has bothered me for a long time, here is the code that reproduces the error:

```
#include
#include
#include
#include
#include
#include
#include

using namespace std;

static auto exec = std::make_shared(
100,
std::make_shared("Exec."));
atomic a = 0;

folly::coro::Generator::iterator iter;

void foo () {
iter++;
}

folly::coro::Task fooTask () {
iter++;
co_return;
}

folly::coro::Generator
gene (int h) {
atomic i = 0;
atomic f = false;

for (; i < h; i++) {
cout << i << endl;
co_yield i;
}
if (f) {
cout << "double exec";
}
f = true;
cout << "complete" </dev/null || break;done
```
to reproduce.

Build and run the supplied reproducer, including the repeated shell loop, then inspect folly/experimental/coro/Generator.h, folly/experimental/coro/Task.h, and CPUThreadPoolExecutor.h. Trace the interaction between the generator iterator, scheduled tasks, and executor threads. Done means the reproducer no longer intermittently segfaults under repeated execution.

Written by the indexing model from the issue text.