[UnindexedFileIndexer] Do not try to add nonexistant file to index

Summary:
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

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21576