Fix a crash in JSON.parse

Properly set members that are actually array indices
and don't crash when trying to set those.

Task-number: QTBUG-35383
Change-Id: I04d4b65c27e97a2e9db19541ed46ee1bb202f780
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>