our nice threaded parser was not kicking in because it checks the size
of the stream with available(), and this available() implementation was
just returning 0.
This makes for a 5-10% performance improvement

Change-Id: I6516862c9c63136b54ca825dd98e35e6d87d4227
Reviewed-on: https://gerrit.libreoffice.org/57263
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>