[new compiler] Fix crash in tst_qquickpathview

[new compiler] Fix crash in tst_qquickpathview

QQuickPathView's attached object uses a dynamic meta-object. So when installing
bindings - such as a signal handler - on it and we want to determine the
property cache for it, we must not use the QQmlEnginePrivate::cache overload
that takes a QObject pointer, as that will - rightly so - return a null
pointer. Instead in this case it is okay to go with the static type overload,
by using the meta-object of the attached type known at compile time.  This is
consistent with the VME code path.

Change-Id: Iffc64f7ee4677c6cc60d071b80f4032cf8037ab7
Reviewed-by: Lars Knoll <lars.knoll@digia.com>