in remainingSize consider that its is possible to seek past the end

Change-Id: I1652244d5515629f1cd8f15f4c5b15f139dba0aa
(cherry picked from commit 6a1f3189)
Reviewed-on: https://gerrit.libreoffice.org/17158Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>

tools/source/stream/stream.cxx

@@ -1464,7 +1464,7 @@ sal_uInt64 SvStream::remainingSize()
 {
     sal_uInt64 const nCurr = Tell();
     sal_uInt64 const nEnd = Seek(STREAM_SEEK_TO_END);
-    sal_uInt64 nMaxAvailable = nEnd-nCurr;
+    sal_uInt64 nMaxAvailable = nEnd > nCurr ? (nEnd-nCurr) : 0;
     Seek(nCurr);
     return nMaxAvailable;
 }