MDEV-8066 Crash on unloading semisync_master plugin

Bug was that ReplSemiSyncMaster::commitTrx() was waiting on a condition
for state to change, but didn't take into account that one could have
disabled semi-sync during the wait.

plugin/semisync/semisync_master.cc

@@ -477,6 +477,7 @@ void ReplSemiSyncMaster::add_slave()
 void ReplSemiSyncMaster::remove_slave()
 {
   lock();
+  assert(rpl_semi_sync_master_clients > 0);
   rpl_semi_sync_master_clients--;
 
   /* Only switch off if semi-sync is enabled and is on */
@@ -744,8 +745,10 @@ 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.
+      active_tranxs_ may be NULL if someone disabled semi sync during
+      cond_timewait()
     */
-    assert(thd_killed(NULL) ||
+    assert(thd_killed(NULL) || !active_tranxs_ ||
            !active_tranxs_->is_tranx_end_pos(trx_wait_binlog_name,
                                              trx_wait_binlog_pos));