This bug only happens with long sleep()s ( > 5 sec), and in debug version.

Analysis: 
The assertion is caused by nested thd_wait_begin() calls, which is not an expected condition.
-  "outer" thd_wait_begin()) , in Item_func_sleep::val_int()
-  "inner" thd_wait_begin() in Interruptible_wait::wait(). This function periodically checks whether connection is still valid, via THD::is_connection(), which ends up  calling vio_io_wait()  with timeout parameter set to  0.


Fix is not to call thd wait callback in vio_io_wait(), if timeout parameter is 0. There is no "waiting" in this case.