cex: fix a crash

Unfortunately, this does not fix the crash of test 247, it's unrelated.

commit f3bc4dc154456ecbf0aa7f65c61fbfe25c646452
Author: Akim Demaille <[email protected]>
Date:   Sat May 16 15:21:59 2020 +0200

    cex: fix a crash
    
    * src/state-item.c (init_state_items): If the rule has no reductions
    at all, don't read at all in its list of reduced rules.

diff --git a/src/state-item.c b/src/state-item.c
index a428c875..167e9ac1 100644
--- a/src/state-item.c
+++ b/src/state-item.c
@@ -150,7 +150,7 @@ init_state_items (void)
           state_item_set (sidx, s, s->items[j]);
           state_item *si = state_items + sidx;
           const rule *r = item_rule (si->item);
-          if (red->rules[rule_search_idx] < r)
+          if (rule_search_idx < red->num && red->rules[rule_search_idx] < r)
             ++rule_search_idx;
           if (rule_search_idx < red->num && r == red->rules[rule_search_idx])
             {