fix ejabberd_system_monitor:s2s_out_info/1 error

large heap watchdog does not working caused by following error:
```
Error in process <0.4634.4> on node 'ejabberd@localhost' with exit value: {badarg,[{erlang,byte_size,[[83,50,83,32,99,111,110,110,101,99,116,105,111,110,58,32,102,114,111,109,32,"conference.xmpp.jp",32,116,111,32,"tigase.im"]],[]},{ejabberd_system_monitor,s2s_out_info,1,[{file,"src/ejabberd_system_monitor.erl"},{line,248}]},{ejabberd_system_monitor,process_large_heap...
```