annotate S_ithread_free mutex use

annotate S_ithread_free mutex use

threads.xs:276:13: warning: releasing mutex 'thread->mutex' that was not held [-Wthread-safety-analysis]
                  MUTEX_UNLOCK(&thread->mutex);

threads.xs:282:5: warning: releasing mutex 'thread->mutex' that was not held [-Wthread-safety-analysis]
          MUTEX_UNLOCK(&thread->mutex);

threads.xs:394:1: warning: mutex 'thread->mutex' is still held at the end of
      function [-Wthread-safety-analysis]
}

threads.xs:677:1: warning: mutex 'thread->mutex' is still held at the end of
      function [-Wthread-safety-analysis]
}