call XIndexAccess.getByIndex() with an invalid index, just like
OEnumerationByName.nextElement() doesn't.

Patch by: me

(cherry picked from commit efa52a41)

Change-Id: Id49e45c18ed00de499cfd93e0945cecaed788ae4
Reviewed-on: https://gerrit.libreoffice.org/41574Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>