SCHEDULER DROPS EVENTS

Problem: On a semi sync enabled server (Master/Slave),
if event scheduler drops an event after completion,
server crashes.

Analaysis: If an event is created with "ON COMPLETION
NOT PRESERVE" clause, event scheduler deletes the event
upon event completion(expiration) and the thread object
will be destroyed. In the destructor of the thread object,
mysys_var member is set to zero explicitly. Later from
the same destructor call(same execution path),
incase of semi sync enabled server, while cleanup is called,
THD::mysys_var member is accessed by THD::enter_cond()
function which causes server to crash.

Fix: mysys_var should not be explicitly set to zero and
also it is not required.

sql/sql_class.cc:
  mysys_var should not be explicitly set to zero.