ofz#14469 null deref

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/71412
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/filter/graphicfilter.cxx

@@ -1442,7 +1442,9 @@ Graphic GraphicFilter::ImportUnloadedGraphic(SvStream& rIStream, sal_uInt64 size
     ErrCode nStatus = ImpTestOrFindFormat("", rIStream, nFormat);
 
     rIStream.Seek(nStreamBegin);
-    const sal_uInt32 nStreamLength( sizeLimit ? sizeLimit : rIStream.remainingSize());
+    sal_uInt32 nStreamLength(rIStream.remainingSize());
+    if (sizeLimit && sizeLimit < nStreamLength)
+        nStreamLength = sizeLimit;
 
     OUString aFilterName = pConfig->GetImportFilterName(nFormat);
     OUString aExternalFilterName = pConfig->GetExternalFilterName(nFormat, false);