Fix merge.test failure

- Problem: mysql_admin_table() calls open_temporary_tables(). This 
  causes assertion failure, because mysql_execute_command() has 
  already called open_temporary_tables()
- Solution: call close_thread_tables() at the start of 
  mysql_admin_table(), like mysql-5.6 does 

mysql-test/r/merge.result

@@ -2262,7 +2262,7 @@ CREATE TABLE m1(a int)engine=merge union=(t1,t2,t3,t4,t5,t6,t7);
 SELECT 1 FROM m1;
 1
 HANDLER m1 OPEN;
-ERROR HY000: Table storage engine for 'm1' doesn't have this option
+ERROR HY000: Storage engine MRG_MyISAM of the table `test`.`m1` doesn't have this option
 DROP TABLE m1,t1,t2,t3,t4,t5,t6,t7;
 SELECT 1 FROM m1;
 ERROR 42S02: Table 'test.m1' doesn't exist
@@ -3421,7 +3421,7 @@ CREATE TABLE t2 (c1 int);
 CREATE TABLE t3 (c1 int) ENGINE = MERGE UNION (t1,t2);
 START TRANSACTION;
 HANDLER t3 OPEN;
-ERROR HY000: Table storage engine for 't3' doesn't have this option
+ERROR HY000: Storage engine MRG_MyISAM of the table `test`.`t3` doesn't have this option
 DROP TABLE t1, t2, t3;
 # Connection default.
 # Disconnecting con1, all mdl_tickets must have been released.

sql/sql_admin.cc

@@ -352,6 +352,14 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
 
   mysql_ha_rm_tables(thd, tables);
 
+  /*
+    Close all temporary tables which were pre-open to simplify
+    privilege checking. Clear all references to closed tables.
+  */
+  close_thread_tables(thd);
+  for (table= tables; table; table= table->next_local)
+    table->table= NULL;
+
   for (table= tables; table; table= table->next_local)
   {
     char table_name[SAFE_NAME_LEN*2+2];