migration/colo: fix use after free of local_err

local_err is used again in secondary_vm_do_failover() after
replication_stop_all(), so we must zero it. Otherwise try to set
non-NULL local_err will crash.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200324153630.11882-5-vsementsov@virtuozzo.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/colo.c

@@ -93,6 +93,7 @@ static void secondary_vm_do_failover(void)
     replication_stop_all(true, &local_err);
     if (local_err) {
         error_report_err(local_err);
+        local_err = NULL;
     }
 
     /* Notify all filters of all NIC to do checkpoint */