bnc#636128 DOC filter: fix handling of FFData.cch

(cherry picked from commit dfa26e6d489a2c5bd79652450a9f27343008d37f)
(cherry picked from commit 2ad1f195b875f2c1052c8ffab79602b9f6871ccb)

Change-Id: Ia1a3a4ef5e61d8ed2a3bde689f3d2101525efc46
Reviewed-on: https://gerrit.libreoffice.org/12257
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>