tst_qquicktextinput/data::maxLength(): Fix assert about zero-width window.

Occurring with ANGLE-builds.

Task-number: QTBUG-32541
Change-Id: Ib48f0c802dd3242a227f8c5d7fda2fccca71403a
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>