MDEV-32614 LeakSanitizer errors in copy_data_between_tables

The memory leak occurs on error when backup_reset_alter_copy_lock fails
with timeout. This leads to the alter rollback, but flush_unused is not
called.

Move table flushing on error handling to a single place and mind more
possible failures this time.

mysql-test/main/alter_table_online_debug.test

@@ -2066,6 +2066,41 @@ select * from t1;
 drop table t1;
 
 
+--echo # MDEV-32614 LeakSanitizer errors in copy_data_between_tables
+create table t (a int, b int) engine=aria;
+insert into t select seq, seq from seq_1_to_5;
+backup stage start;
+
+--connect (con_lock,localhost,root,,)
+set lock_wait_timeout= 1;
+
+set debug_sync='copy_data_between_tables_before_reset_backup_lock wait_for continue';
+send alter table t add index (b), algorithm=copy, lock=none;
+
+--connection default
+backup stage block_commit;
+
+set debug_sync='now signal continue';
+
+--connection con_lock
+--error ER_LOCK_WAIT_TIMEOUT
+--reap
+# --echo # error $mysql_errno
+
+set debug_sync='copy_data_between_tables_before_reset_backup_lock wait_for continue';
+send alter table t add index (a), algorithm=copy, lock=none;
+
+--connection default
+backup stage end;
+set debug_sync='now signal continue';
+
+--connection con_lock
+--reap
+--disconnect con_lock
+--connection default
+drop table t;
+
+
 eval set global default_storage_engine= $default_storage_engine;
 
 --disconnect con1