Use a new fast parser instance for each XML fragment.

Otherwise a crash ensues when the threaded XML parsing kicks in.

Change-Id: Ic41e5a29bbb860d7b63b70f2f0d8896264d9d53e
(cherry picked from commit dc93074f71f91efd8a615ad8f1a5289deb210b75)
Reviewed-on: https://gerrit.libreoffice.org/10003
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>