MWL#89: Cost-based choice between Materialization and IN->EXISTS transformation

Fixed a crash in Item_equal::fix_length_and_dec that was a result of the
architecture of the MWL#89 subquery optimization.

The injection of IN->EXISTS predicates for subqueries happened inside
make_join_statistics, after constant substitution, but before multiple
equality substitution done by substitute_for_best_equal_field. As a
result, when we called fix_fields for the WHERE clause after the injection
of IN->EXISTS predicates, Item_equal was not fixed, and it was in a state
not anticipated by Item_equal::fix_length_and_dec - the Item_equal
containted only a constant, and no fields at all.

The fix takes into account this new possible state when calling fix_fields.

sql/item_cmpfunc.cc

@@ -5649,6 +5649,9 @@ longlong Item_equal::val_int()
   Item_field *item_field;
   if (cond_false)
     return 0;
+  /* If there is a single constant and no fields, the equality is TRUE. */
+  if (const_item && !fields.elements)
+    return 1;
   List_iterator_fast<Item_field> it(fields);
   Item *item= const_item ? const_item : it++;
   if ((null_value= item->null_value))
@@ -5669,6 +5672,15 @@ longlong Item_equal::val_int()
 void Item_equal::fix_length_and_dec()
 {
   Item *item= get_first(NULL);
+  if (!item)
+  {
+    /*
+      If there are no fields, there must be at least a constant, in which
+      case Item_equal::val_int evaluates to TRUE.
+    */
+    DBUG_ASSERT(const_item);
+    return;
+  }
   eval_item= cmp_item::get_comparator(item->result_type(),
                                       item->collation.collation);
 }