MDEV-10944: GALERA log-slave-updates FAIL after upgrading from 10.1.17 to 10.1.18

thd->variables.option_bits need to be restored after
plugin_thdvar_init() during post initialization of
wsrep threads.

sql/sql_plugin.cc

@@ -4327,7 +4327,13 @@ void wsrep_plugins_post_init()
   {
     if (IF_WSREP(thd->wsrep_applier,1))
     {
+      // Save options_bits as it will get overwritten in plugin_thdvar_init()
+      ulonglong option_bits_saved= thd->variables.option_bits;
+
       plugin_thdvar_init(thd);
+
+      // Restore option_bits
+      thd->variables.option_bits= option_bits_saved;
     }
   }