assert() in semisync.master.cc

In 10.0 plugin/semisync/semisync_master.cc has this assert:

void ReplSemiSyncMaster::remove_slave()

{

  lock();

  assert(rpl_semi_sync_master_clients > 0);

  rpl_semi_sync_master_clients--;

In 10.1 it was removed during the merge. It should be added back. Make sure that all semisync-related tests pass with it.