make sure that we don't try to access an already deleted files

This fixes some file source not found warnings, see #4733 

@PVince81 yesterday we introduced some more warnings with https://github.com/owncloud/core/commit/d2840a045d8d4e5996c847e8bbf93e80c111fdac they are also fixed in this PR

@icewind1991 can you also have a look, especially at my changes in lib/private/files/cache/updater.php. Thanks! I think it should be OK to call correctFolder() and correctFolderSize() on the parent, since the child is already deleted.