sw: do more input validation in SwWW8ImplReader::Read_And

(to fix up f2945255df273404ee2457dcf761cb8f334b732b)

Change-Id: Ie20fb9db4515b9737322ec7224ecd7e411d31c03
(cherry picked from commit ae2e7ad276acb9394691a9d4a702ed7a6b07b508)
Reviewed-on: https://gerrit.libreoffice.org/10141
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>