[BUGFIX] Use correct file data variable in Indexer

The processChangedAndNewFiles() method in the Indexer class now uses
the $fileIndexEntry variable for retrieving a file object from the
resource factory instead of the invalid $data variable which is NULL.

Resolves: #54312
Releases: 6.2
Change-Id: I3b9c2ce99f7b4b7c575cc4055c02912c306ed789
Reviewed-on: https://review.typo3.org/26339
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-by: Markus Klein
Tested-by: Markus Klein