valgrind + bff: don't add 0xffff oldcode

Invalid read of size 1
   at 0x9DC6247: GIFLZWDecompressor::AddToTable(unsigned short, unsigned short) (decode.cxx:117)

Change-Id: I9cea8dcd4a5eeaa1e793b9be96f82854b9d088af
Reviewed-on: https://gerrit.libreoffice.org/10906
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>