Correctly signal end of input file from RootIOAlgExt

!4890 (merged) missed to flag the file as completely read when using RootIOAlgExt (see discussion on mattermost)

This MR adds the missing line.

For the record, this line should have been added in !4890 (merged) when the usage of XMLSummarySvc with RootIOAlg was fixed.

To-do:

• add a test