Post-merge fixes:

- Fix derived_view.test to work, and enable it
- Let subselect*.test do "DROP TABLE IF EXISTS" before they attempt to create the table.

mysql-test/r/subselect4.result

-drop table if exists t1,t2,t3,t4,t5,t6;
+drop table if exists t0,t1,t2,t3,t4,t5,t6;
 drop view if exists v1, v2;
 set @subselect4_tmp= @@optimizer_switch;
 set optimizer_switch='semijoin=on,firstmatch=on,loosescan=on';

mysql-test/r/subselect_cache.result

+drop table if exists t0,t1,t2,t3,t4,t5,t6,t7,t8,t9;
 set optimizer_switch='subquery_cache=on';
 create table t1 (a int, b int);
 insert into t1 values (1,2),(3,4),(1,2),(3,4),(3,4),(4,5),(4,5),(5,6),(5,6),(4,5);

mysql-test/t/disabled.def

@@ -20,4 +20,3 @@ archive-big              : Bug#11817185 2011-03-10 Anitha Disabled since this le
 log_tables-big           : Bug#11756699 2010-11-15 mattiasj report already exists
 mysql_embedded           : Bug#12561297 2011-06-15 New test failing on all platforms
 tablespace               : disabled in MariaDB (no TABLESPACE table attribute)
-derived_view            : SergeyP

mysql-test/t/subselect4.test

 # General purpose bug fix tests go here : subselect.test too large
 
 --disable_warnings
-drop table if exists t1,t2,t3,t4,t5,t6;
+drop table if exists t0,t1,t2,t3,t4,t5,t6;
 drop view if exists v1, v2;
 --enable_warnings
 

mysql-test/t/subselect_cache.test

+--disable_warnings
+drop table if exists t0,t1,t2,t3,t4,t5,t6,t7,t8,t9;
+--enable_warnings
 set optimizer_switch='subquery_cache=on';
 
 create table t1 (a int, b int);

sql/item_subselect.cc

@@ -2681,9 +2681,14 @@ bool subselect_union_engine::no_rows()
 void subselect_uniquesubquery_engine::cleanup()
 {
   DBUG_ENTER("subselect_uniquesubquery_engine::cleanup");
-  /* Tell handler we don't need the index anymore */
-  if (tab->table->file->inited)
-    tab->table->file->ha_index_end();
+  /* 
+    Note for mergers: we don't have to, and actually must not de-initialize
+    tab->table->file here.
+    - We don't have to, because free_tmp_table() will call ha_index_or_rnd_end
+    - We must not do it, because tab->table may be a derived table which 
+      has been already dropped by close_thread_tables(), while we here are
+      called from cleanup_items()
+  */
   DBUG_VOID_RETURN;
 }
 

sql/sql_select.cc

@@ -14793,6 +14793,7 @@ free_tmp_table(THD *thd, TABLE *entry)
 
   if (entry->file && entry->created)
   {
+    entry->file->ha_index_or_rnd_end();
     if (entry->db_stat)
       entry->file->ha_drop_table(entry->s->table_name.str);
     else