Bug#24791: Union with AVG-groups generates wrong results

Patch appled after doing a pull from the team tree. Additional tests had to be
fixed


mysql-test/r/union.result:
  Bug 24791
  
  The tests for temporary tables have been fixed. Since the call to 
  display_length(Item) was removed from the constructor for Item_type_holder, items 
  in temporary tables keep the original values of the items, rather than the magic
  numbers supplied by display_length.

mysql-test/r/union.result

@@ -554,7 +554,7 @@ aa
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` varbinary(20) NOT NULL default ''
+  `a` varbinary(2) NOT NULL default ''
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 SELECT 12 as a UNION select 12.2 as a;
@@ -655,7 +655,7 @@ f
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `f` varbinary(24) default NULL
+  `f` varbinary(12) default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 SELECT y from t2 UNION select da from t2;