MDEV#6296: runtime adjustment of wsrep_slave_threads creates

threads but never removes them

When wsrep_slave_threads is changed at runtime, the old "change"
value is not taken into account and thus successive SETs can
make wsrep_slave_threads value inconsistent with the actual
number of applier threads present.

sql/wsrep_var.cc

@@ -453,7 +453,7 @@ void wsrep_node_address_init (const char* value)
 bool wsrep_slave_threads_check (sys_var *self, THD* thd, set_var* var)
 {
   mysql_mutex_lock(&LOCK_wsrep_slave_threads);
-  wsrep_slave_count_change = var->value->val_int() - wsrep_slave_threads;
+  wsrep_slave_count_change += (var->value->val_int() - wsrep_slave_threads);
   mysql_mutex_unlock(&LOCK_wsrep_slave_threads);
 
   return 0;