facepalm: nAvailHeight is not derived from allocation Width

Change-Id: Ic99c6519b02c546e0d3574cb756e3fb172d1ff52
(cherry picked from commit 36490fff3f4d1b4a7e4b7902a866abb177a0bf54)
Reviewed-on: https://gerrit.libreoffice.org/11023
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>