os_get_nthreads() return the total number of threads. Not the number of workers.

E.g. the stats thread is not a worker thread.

This crashes if new thread is added with VLIB_REGISTER_THREAD()      

uword thread_index = 0;

      int index;

      const uword nthreads = os_get_nthreads ();

      u32 *vec_icmp_bi = NULL;

      for (thread_index = 0; thread_index < nthreads; ++thread_index)

        {

          ip6_reass_per_thread_t *rt = &rm->per_thread_data[thread_index];

          clib_spinlock_lock (&rt->lock);