Check thread count for negative; realloc->reallocarray

static int

threadpool_start_threads(threadpool_t *pool, int n)

{

  if (n < 0)

    return -1;

  tor_mutex_acquire(&pool->lock);

  if (pool->n_threads < n)

    pool->threads = tor_realloc(pool->threads, sizeof(workerthread_t*)*n);

    pool->threads = tor_reallocarray(pool->threads,

                                     sizeof(workerthread_t*), n);

  while (pool->n_threads < n) {

    void *state = pool->new_thread_state_fn(pool->new_thread_state_arg);