vm.c: fix `mrb_ci_kidx`.

It used to return wrong value for 14 positional arguments.

src/vm.c

@@ -394,10 +394,9 @@ mrb_funcall_id(mrb_state *mrb, mrb_value self, mrb_sym mid, mrb_int argc, ...)
 }
 
 static mrb_int
-mrb_ci_kidx(mrb_callinfo *ci)
+mrb_ci_kidx(const mrb_callinfo *ci)
 {
-  if (ci->nk == 0) return -1;
-  return (ci->n % 14) + 1;
+  return (ci->n == CALL_MAXARGS) ? 2 : ci->n + 1;
 }
 
 static mrb_int