`cobalt::thread` doesn't release mutex

Following code:

```
#include

namespace cobalt = boost::cobalt;

cobalt::thread emptyThread()
{
co_return;
}

int main(int /*argc*/, char** /*argv*/)
{
auto thread = emptyThread();
thread.join();
return 0;
}
```

Built on Windows 10 64-bit using MSVC 17.9.0 (cl 19.39.33519 for x64) with following command:
`cl /Femain.exe main.cpp /std:c++20 /MDd /EHsc -D_WIN32_WINNT=0x0601 /external:W0 /external:IC:\Users\chodorowskip\repo\deps\vc143\boost-1.84.0\include\boost-1_84 /link /DEBUG /LIBPATH:C:\Users\chodorowskip\repo\deps\vc143\boost-1.84.0\lib libboost_cobalt-vc143-mt-gd-x64-1_84.lib`

Call aborts and gives following error message in console:
`D:\a\_work\1\s\src\vctools\crt\github\stl\src\mutex.cpp(49): mutex destroyed while busy`

This error doesn't seem to happen when thread is suspended at least once. For example using the following code:

```
#include

#include
#include

namespace asio = boost::asio;
namespace cobalt = boost::cobalt;

struct Awaitable
{
bool await_ready()
{
return false;
}
template
void await_suspend(std::coroutine_handle handle)
{
auto executor = handle.promise().get_executor();
auto h = cobalt::unique_handle(handle.address());
asio::post(executor, std::move(h));
}
void await_resume() {}
};

cobalt::thread emptyThread()
{
co_await Awaitable{};
}

int main(int /*argc*/, char** /*argv*/)
{
auto thread = emptyThread();
thread.join();
return 0;
}
```