Doctest update for isl 0.17

doc/tutorial.rst

@@ -563,8 +563,9 @@ relation to loop nesting. For example, it's perfectly possible to request
     #define lid(N) ((int) get_local_id(N))
     ...
       for (int i_inner = 0; i_inner <= 15; ++i_inner)
-        for (int i_outer = 0; i_outer <= -1 + -1 * i_inner + ((15 + n + 15 * i_inner) / 16); ++i_outer)
-          a[i_inner + i_outer * 16] = 0.0f;
+        if (-1 + -1 * i_inner + n >= 0)
+          for (int i_outer = 0; i_outer <= -1 + -1 * i_inner + ((15 + n + 15 * i_inner) / 16); ++i_outer)
+            a[i_inner + i_outer * 16] = 0.0f;
     ...
 
 Notice how loopy has automatically generated guard conditionals to make