Fix cases where mark() would access uninitialized memory

Change-Id: I4e07e20d30ba57759a0ece1c298a02b098718b33
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix grouped property access for non-value type properties

If we don't have a value type, read the grouped property as QObject
instance and populate that instead.

Fixes tst_qqmllanguage::deepProperty

Change-Id: I9a8d5f929a9c0ebb39461863f6a93b3f35274f97
Reviewed-by: Lars Knoll <lars.knoll@digia.com>