MDEV-7568: STOP SLAVE crashes the server

The order of initialisation during server startup was incorrect. The slave
threads were started before the parallel replication worker thread pool was
initialised, allowing a race where uninitialised data could be accessed.

sql/slave.cc

@@ -371,6 +371,9 @@ int init_slave()
   if (run_slave_init_thread())
     return 1;
 
+  if (global_rpl_thread_pool.init(opt_slave_parallel_threads))
+    return 1;
+
   /*
     This is called when mysqld starts. Before client connections are
     accepted. However bootstrap may conflict with us if it does START SLAVE.
@@ -404,9 +407,6 @@ int init_slave()
     goto err;
   }
 
-  if (global_rpl_thread_pool.init(opt_slave_parallel_threads))
-    return 1;
-
   /*
     If --slave-skip-errors=... was not used, the string value for the
     system variable has not been set up yet. Do it now.