since...

commit af84fc9d
Date:   Tue Apr 23 15:48:41 2019 +0200

    lazy image loading shouldn't read the entire .xls file (tdf#124828)

nLength is just an unchecked value in the dff stream, it might not be sane
so limit it to the max len of the stream

Change-Id: Ia8a2830478952afe1317b5cd795f35059d9b380a
Reviewed-on: https://gerrit.libreoffice.org/71413
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>