GHSA-w59h-378f-2frm: Unsound sending of non-Send types across threads in threadalone

Affected versions can run the Drop impl of a non-Send type on a different thread than it was created on.

The flaw occurs when a stderr write performed by the threadalone crate fails, for example because stderr is redirected to a location on a filesystem that is full, or because stderr is a pipe that has been closed by the reader.

Dropping a non-Send type on the wrong thread is unsound. If used with a type such as a pthread-based MutexGuard, the consequence is undefined behavior. If used with Rc, there would be a data race on the reference count, which is likewise undefined behavior.