This routine destroys a mutex attributes object-that is, the
    object becomes uninitialized. Call this routine when your program
    no longer needs the specified mutex attributes object.

    After this routine is called, the Threads Library may reclaim
    the storage used by the mutex attributes object. Mutexes that
    were created using this attributes object are not affected by the
    destruction of the mutex attributes object.

    The results of calling this routine are unpredictable, if the
    attributes object specified in the attr argument does not exist.