CAN RESULTS IN SERVER EXIT

Problem & Analysis
==================

When slave sends COM_REGISTER_SLAVE, then slave information is stored
in slave_list on master.Slave information in list holds the reference
of dump thread's THD.

Slave is unregistered on master after completing a binlog dump (i.e
after serving COM_BINLOG_DUMP) by calling unregister_slave().

When slave calls COM_REGISTER_SLAVE and gets disconnected. While going
down it sends COM_QUIT to the master. COM_QUIT causes the master dump
thread to go down. But it does not unregister the slave on master,
which leaves slave_list at master in an inconsistent state.

SHOW SLAVE HOSTS access slave_list to get the slave information.
Accessing invalid memory related to THD stored in slave list results
in server abnormal exits.

Fix
===
The fix for the above problem is to remove the references of the THD
pointer from the slave_list when the thread is shutting down. This is
done by calling unregister_slave() from THD::~THD() function.

Patch backports mutex_lock guards to 5.6 from 5.7.

RB#23245