MDEV-7201: parallel threads resizing - potential race condition to access freed memory

pool->threads is freed before being reassigned the new pool.

Although not really a memory barrier I though it prudent to keep the pool
thread count to be the lower of the old/new thread list before the new threads
is allocated.

sql/rpl_parallel.cc

@@ -1007,6 +1007,7 @@ rpl_parallel_change_thread_count(rpl_parallel_thread_pool *pool,
                                  uint32 new_count, bool skip_check)
 {
   uint32 i;
+  rpl_parallel_thread **old_list= NULL;
   rpl_parallel_thread **new_list= NULL;
   rpl_parallel_thread *new_free_list= NULL;
   rpl_parallel_thread *rpt_array= NULL;
@@ -1111,10 +1112,14 @@ rpl_parallel_change_thread_count(rpl_parallel_thread_pool *pool,
     }
   }
 
-  my_free(pool->threads);
+  old_list= pool->threads;
+  if (new_count < pool->count)
+    pool->count= new_count;
   pool->threads= new_list;
-  pool->free_list= new_free_list;
+  if (new_count > pool->count)
     pool->count= new_count;
+  my_free(old_list);
+  pool->free_list= new_free_list;
   for (i= 0; i < pool->count; ++i)
   {
     mysql_mutex_lock(&pool->threads[i]->LOCK_rpl_thread);