Fix Undefined index: storage in share.php

There parameter was removed when the code was cleaned up:
{"app":"PHP","message":"Undefined index: storage at
E:\\ownCloud\\owncloud\\lib\\private\\share\\share.php#1160",
"level":0,"time":"2014-04-09T12:56:23+00:00"}
Merge pull request #8128 from owncloud/fix/undefined-index-storage

Fix Undefined index: storage in share.php