Fixes valgrind warning like below when executing tst_QXmlQuery::copyConstructor()
  Conditional jump or move depends on uninitialised value(s)
    at: QPatternist::NodeIndexStorage::operator!=(QPatternist::NodeIndexStorage const&) const (qabstractxmlnodemodel.cpp:1220)
    by: QXmlItem::operator=(QXmlItem const&) (qabstractxmlnodemodel.cpp:1228)

Reason for the warning is that QPatternist::NodeIndexStorage::operator!=
accesses all fields of NodeIndexStorage, which are all not intialized in
every execution path of QXmlItem::QXmlItem(const QVariant &) and class
QPatternist::Item constructors.

Fixed by adding NodeIndexStorage::reset() function that resets all fields
and put a call to that function where NodeIndexStorage objects were
previously incompletely initialized. Note that unfortunately class
NodeIndexStorage cannot have a default constructor, because it is used as
a union field.

Change-Id: I758df57551ec49ce8c8b357794177b4e6c454d2f
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Dmitry Trofimov
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>