Problem: While some active transaction are waiting for ack from Slave,
         executing 'RESET MASTER' is leaving Master's semisync
         in bad state.

Analysis: In ReplSemiSyncMaster::commitTrx, when transaction is
          waiting for an acknowledgement from the Slave, before
          it goes into the wait state, it increases
          rpl_semi_sync_master_wait_sessions status variable to indicate
          the number of sessions waiting for the acknowledgements. Also
          before it goes into the wait state, it releases LOCK_binlog_ lock
          to avoid any hang issues for the other sessions which are looking
          for LOCK_binlog_. When the thread releases LOCK_binlog_ lock and
          waiting for the acknowledgement, there is a possibility that
          some other thread which is executing 'RESET MASTER' can get the lock
          and reset all the status variables. rpl_semi_sync_master_wait_sessions
          is one of such status variables. After the wait is over in
          ReplSemiSyncMaster::commitTrx (either the timeout or received acknowledgement
          from Slave), the logic is decreasing rpl_semi_sync_master_wait_sessions)
          without checking whether 'RESET MASTER' was executing by any other parallel
          thread. This leads invalid value for rpl_semi_sync_master_wait_sessions
          status variable. Any other logic which is trying to use this status variable
          will lead to unexpected results in the server.
Fix: Check the value of rpl_semi_sync_master_wait_sessions before decrementing
     it and decrement it only if it is non-zero value.