Preventing shared sem to be used again

The shared semaphore is closed but it's not assigned to null.
That's required because the logic check the semaphore status if it's not null. For this reason, we are getting a core some times.

src/libfaketime.c

@@ -628,6 +628,7 @@ static void ft_cleanup (void)
   if (shared_sem != NULL)
   {
     sem_close(shared_sem);
+    shared_sem = NULL;
   }
 #ifdef FAKE_PTHREAD
   if (pthread_rwlock_destroy(&monotonic_conds_lock) != 0) {