Fix access to uninitialised memory that might lead to crashes

Fix access to uninitialised memory that might lead to crashes

QMetaObject is POD, so its constructor is implicit and trivial
(doesn't initialise anything). QAbstractDynamicMetaObject doesn't add
a constructor, so the QMetaObject sub-object remains
uninitialised. The users of either class must ensure they initialise
the members if they will be accessed.

Change-Id: Ibb7f55ff23b78afb1fcb87382b30c8a28804c028
Task: QTBUG-23214
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>