Assert on (! shutdown_), function schedule_from_remote

While investigating another case just found this assert fired.
Boost fiber from develop branch.

The toy example to reproduce (some time it needs several runs):

```cpp
#include
#include
#include
#include
#include

using namespace std::chrono_literals;

int main()
{
boost::fibers::mutex mtx;
boost::fibers::condition_variable cond;
std::atomic stopping;

std::vector pool;
for (size_t i = 0; i < 10; i++)
{
auto th = std::thread([&] ()
{
boost::fibers::use_scheduling_algorithm();
auto f1 = boost::fibers::fiber([&] () {
int i = 0;
while (!stopping.load())
{
std::cout << "trying to get lock" << std::endl;
std::unique_lock l(mtx);
std::this_thread::sleep_for(100ms);
std::cout << i++ << " waiting" << std::endl;
cond.wait_for(l, 15ms);
std::cout << "worker" << std::endl;
}
});
f1.join();
});
pool.push_back(std::move(th));
}

auto th2 = std::thread([&] ()
{
while (!stopping.load())
{
{
std::unique_lock l(mtx);
cond.notify_all();
}
std::this_thread::sleep_for(1ms);
}
});

std::this_thread::sleep_for(200ms);
stopping.store(true);

for (auto& th : pool)
{
th.join();
}
th2.join();

return 0;
}
```

possible output:
```
trying to get lock
trying to get lock
trying to get lock
trying to get lock
trying to get lock
trying to get lock
trying to get lock
trying to get lock
trying to get lock
trying to get lock
0 waiting
0 waiting
0 waiting
worker
0 waiting
0 waiting
0 waiting
0 waiting
Assertion failed: (! shutdown_), function schedule_from_remote, file libs/fiber/src/scheduler.cpp, line 227.
```