select from I_S

Problem:
========
When applier thread tries to access 'variable_name' of
INFORMATION_SCHEMA.SESSION_VARIABLES table through triggers, it results in an
abnormal exit of slave server.

Analysis:
========
At the time of replication of stored routines and triggers, their associated
security context will be sent by the master. The applier thread on the slave
server will use this information to set the required security context for the
execution of stored routines and triggers. This is achieved as follows.

->The stored routine object has a member named 'm_security_ctx' which holds the
  security context received from master.
->The applier thread's security_ctx is stored into a 'backup' object.

->Set the applier thread's security_ctx to 'm_security_ctx'.

->Upon the completion of stored routine execution restore the original security
  context of applier thread from the backup.

During the above process the 'm_security_ctx' object is not initialized
properly. Hence the 'external_user' of 'm_security_ctx' has invalid value for
this variable and accessing this variable results in abnormal exit of server.

Fix:
===
Invoke the Security_context::init() call from the constructor of stored routine
so that 'm_security_ctx' gets initialized properly.