Yet another one undefined comparison of 'const' and a 'non-const' iterators (hope the last one)

Change-Id: Iefcd1f5561e5a8cac4152f16e72e8f628e9204a5
Reviewed-on: https://gerrit.libreoffice.org/8223
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>