MDEV-6833: SIGSEGV on shutdown with non-default wsrep_slave_threads

thd->variables' table_plugin & tmp_table_plugin should be set to
NULL for wsrep system threads.
Also made a minor change to skip checking of wsrep options if
wsrep_on is not set.

sql/mysqld.cc

@@ -4981,8 +4981,10 @@ a file name for --log-bin-index option", opt_binlog_index_name);
   }
   plugins_are_initialized= TRUE;  /* Don't separate from init function */
 
-  if (wsrep_check_opts())
-    unireg_abort(1);
+#ifdef WITH_WSREP
+  if (WSREP_ON && wsrep_check_opts())
+    global_system_variables.wsrep_on= 0;
+#endif
 
   /* we do want to exit if there are any other unknown options */
   if (remaining_argc > 1)

sql/sql_plugin.cc

@@ -3098,6 +3098,9 @@ void plugin_thdvar_init(THD *thd)
     intern_plugin_unlock(NULL, old_table_plugin);
     intern_plugin_unlock(NULL, old_tmp_table_plugin);
     mysql_mutex_unlock(&LOCK_plugin);
+  } else {
+    thd->variables.table_plugin= NULL;
+    thd->variables.tmp_table_plugin= NULL;
   }
 
   DBUG_VOID_RETURN;