Avoid using a dangling pointer when removing the last item

When it's the last item that is removed int_thread_hash == hash and we would
still call int_thread_release(&hash) while hash is already freed.  So
int_thread_release would compare that dangling pointer to NULL which is
undefined behaviour.  Instead do already what int_thread_release() would do,
and make the call do nothing instead.

Reviewed-by: Rich Salz <rsalz@openssl.org>

RT: #4155, MR: #1519