nth0/3 is not correct

I tried the current definition of [`nth0/3`](https://github.com/ichiban/prolog/blob/08fb802684ffe7d28dac2888121cce02b282ff18/bootstrap.pl#L241) with Scryer Prolog, and get for example:

<pre>
<b>?- nth0(N, "abc", E).</b>
   N = 0, E = a.
</pre>

Expected:

<pre>
<b>?- nth0(N, "abc", E).</b>
   N = 0, E = a
;  N = 1, E = b
;  N = 2, E = c
;  false.
</pre>