xmerge: use java.nio.ByteBuffer (JDK 1.4+)

Change-Id: I6072bb4699c9cd361e0cfe0a12b46cfc1abfe368
Reviewed-on: https://gerrit.libreoffice.org/11998
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>