- match pthread_getspecific behavior for uninitialized keys
pthread_getspecific() returns nullptr when the key has not been initialized with a pthread_setspecific(). Match this behavior in libfibre, because this is needed in the Rust port.
pthread_getspecific() returns nullptr when the key has not been initialized with a pthread_setspecific(). Match this behavior in libfibre, because this is needed in the Rust port.