MariaDB crashed when using thread pool

When using thread pool and execute 'select sleep(10);', the server crashed.

[How to repeat]

In the my.cnf, set thread_pool_handling=pool-of-threads

Then connect to the server:
MariaDB [(none)]> select sleep(10000);

Then the server crashed with the information below:

mysqld: /home/loushuai/src/mariadb/10.0/sql/threadpool_unix.cc:1283: void tp_wait_begin(THD*, int): Assertion `!connection->waiting' failed.

Program received signal SIGABRT, Aborted.

[Reason]
repeat to enter the tp_wait_begin() in
longlong Item_func_sleep::val_int() function in file sql/item_func.cc

[How to fix]
see the patch in attachment.