Bug#25782811 5.6: WRONG RESULTS WITH MATERIALIZATION,

                        VIEWS

Backport of Bug#21566735 fix.

Analysis
=======
create_ref_for_key() fails due to table not available for
key_use when subquery optimization chooses materialization
strategy and have a ref access key to one of the materialized
table. This is due to JOIN::update_equalities_for_sjm() checks
for real_item() of the materialized table expression with key_use->val
of the primary table expresssion.

Fix
====
When comparing key_use from materialized tables use key_use->
val->real_item() instead of key_use->val.

Author: Ajo Robert

sql/sql_optimizer.cc

@@ -3024,7 +3024,7 @@ bool JOIN::update_equalities_for_sjm()
         uint fieldno= 0;
         while ((old= it++))
         {
-          if (old->real_item()->eq(keyuse->val, false))
+          if (old->real_item()->eq(keyuse->val->real_item(), false))
           {
             /*
               Replace the expression selected from the subquery with the