Bug#17327454:SEMI-SYNC REPLICATION MASTER CRASH WHEN SET

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

@@ -752,7 +752,8 @@ int ReplSemiSyncMaster::commitTrx(const char* trx_wait_binlog_name,
       At this point, the binlog file and position of this transaction
       must have been removed from ActiveTranx.
     */
-    assert(!active_tranxs_->is_tranx_end_pos(trx_wait_binlog_name,
+    assert(!getMasterEnabled() ||
+           !active_tranxs_->is_tranx_end_pos(trx_wait_binlog_name,
                                              trx_wait_binlog_pos));
     
     /* Update the status counter. */