Skip creating an intermediary buffer in XStream_impl::readBytes, and
just read directly into the destination.

This is specifically fixing the performance of queryTypeByDescriptor
when called from a basic macro on a local test file.

This takes my test macro from 17.1s to 16.1s.

Change-Id: Iaa7d38c6a90a3b3f01a4b748c4512dd8fda690c7
Reviewed-on: https://gerrit.libreoffice.org/73374
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>