[UnindexedFileIndexer] Do not try to add nonexistant file to index
A file may be deleted after it has been picked up by the iterator,
causing the fileId to be 0. This is not an error, remove the Q_ASSERT
and silently skip the file.
Reviewers: Baloo, ngraham, astippich, poboiko
Reviewed By: Baloo, ngraham, poboiko
Differential Revision: https://phabricator.kde.org/D21576