Fix service warning

Got these on the remote server when testing server to server sharing with the sync client:
```
{"app":"PHP","message":"Undefined index: service at \/var\/www\/owncloud\/public.php#20","level":3,"time":"2014-06-30T15:27:58+00:00"}
{"app":"PHP","message":"Undefined index: service at \/var\/www\/owncloud\/public.php#20","level":3,"time":"2014-06-30T15:27:58+00:00"}
{"app":"PHP","message":"Undefined index: service at \/var\/www\/owncloud\/public.php#20","level":3,"time":"2014-06-30T15:27:58+00:00"}
{"app":"PHP","message":"Undefined index: service at \/var\/www\/owncloud\/public.php#20","level":3,"time":"2014-06-30T15:27:58+00:00"}
```

This PR fixes that.

Please review @icewind1991 @schiesbn 

We should pay more attention to warnings in our PRs. :smile: