Fixed typo in mysql_admin_table which cused call of
close_unused_temporary_table_instances alwas for the first table
instead of the current table.
Added ASSERT that close_unused_temporary_table_instances should not
remove all instances of user created temporary table.
truncating a temporary table
TRUNCATE expects only one TABLE instance (which is used by TRUNCATE
itself) to be open. However this requirement wasn't enforced after
"MDEV-5535: Cannot reopen temporary table".
Fixed by closing unused table instances before performing TRUNCATE.
