WMF import: restore support for the ImplReadDIB(bFileHeader=false) version

Regression from commit f7799c9317cc3187ae8aaedc36f829d478a59e77
(Resolves: #i124467# add check for image data offset..., 2014-03-28).

Change-Id: I71e78aefca2ca5b07f763ec798226b9bb39a5254
(cherry picked from commit 9984807f17593b97778fc31824c50706ea2e826c)
Reviewed-on: https://gerrit.libreoffice.org/8849
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>