Fix crashes when running tst_qqmlecmascript with MM_AGGRESSIVE_GC

Fix crashes when running tst_qqmlecmascript with MM_AGGRESSIVE_GC

Make sure to initialize the property data (especially setter/getter) in
insertMember() right away to zero, to avoid marking uninitialized objects
in code like this when the GC happens in code like this:

Property *pd = o->insertMember(...)
f = someFunctionThatAllocates();
pd->setSetter(f);

Change-Id: I55e4846c0e5027bb95a1da13df30bcf9eca28645
Reviewed-by: Lars Knoll <lars.knoll@digia.com>