From Jan Iwaszkiewicz

Fix a problem in the PROOF file upload.
The problem starts when uploading a dataset with TProof::kAppend flag set, while the previous dataset is not present on the cluster.


git-svn-id: http://root.cern.ch/svn/root/trunk@15704 27541ba8-7e3a-0410-8455-c3a389f83636