Fix memory leak caused by QV4::ErrorObject

We should destruct QV4::ErrorObject::Data instead of QV4::ErrorObject in
QV4::ErrorObject::destroy() since all the members is stored in
QV4::ErrorObject::Data.

Task-number: QTBUG-42340
Change-Id: Ifff6413c0726591c335a421a5f289c1886f80980
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>