MDEV-9159: Bring back assert in semisync_master.cc

In 10.0 there was an assert to ensure that there were semi
sync clients before removing one, but it was removed in 10.1.
This patch adds the assertion back.

sql/semisync_master.cc

@@ -571,6 +571,7 @@ void Repl_semi_sync_master::add_slave()
 void Repl_semi_sync_master::remove_slave()
 {
   lock();
+  DBUG_ASSERT(rpl_semi_sync_master_clients > 0);
   if (!(--rpl_semi_sync_master_clients) && !rpl_semi_sync_master_wait_no_slave)
   {
     /*