MDEV-4506: Parallel replication.

Delete any left-over deferred_event from rpl_group_info when
deleting the parent object, to protect against memory leaks.

sql/rpl_rli.cc

@@ -1489,6 +1489,7 @@ rpl_group_info::rpl_group_info(Relay_log_info *rli_)
 rpl_group_info::~rpl_group_info()
 {
   free_annotate_event();
+  delete deferred_events;
   mysql_mutex_destroy(&sleep_lock);
   mysql_cond_destroy(&sleep_cond);
 }

sql/rpl_rli.h

@@ -592,7 +592,10 @@ public:
   void cleanup_after_session()
   {
     if (deferred_events)
+    {
       delete deferred_events;
+      deferred_events= NULL;
+    }
   };
 
   /**