RPL_SEMI_SYNC_MASTER_ENABLED OFF.

Problem:
=======
If master is waiting for a reply from slave, at this time
set global rpl_semi_sync_master_enabled=OFF, the master
server will crash.

Analysis:
========
When master is waiting for a reply from slave, at this time
if semi sync is switched off on master, during switch off if
active transactions are present the transactions will be
cleared and "active_tranxs_" variable will be set to NULL.

When the waiting master connection finds that semi sync is
switched of it tries to access "active_tranxs_" without
checking if the transaction list exists or not. Accessing
NULL transaction list causes the crash.

Fix:
===
A check has been added to see a valid list exists before
accessing the "active_tranxs_".

plugin/semisync/semisync_master.cc:
  Added check for the existence of valid 'active_transx_'.