The QDateTimeData &d it's passed is a copy that's about to be
modified; before we do so, we haven't detached so its internals have a
ref-count of two, contradicting an assertion in the non-const
Data::operator->(); so just directly access d.d->m_timezone, since we
know that spec == TimeZone implies !isShort().
Added test that triggered the assertion and now doesn't.
Fixes: QTBUG-99668
Pick-to: 6.3 6.2 6.2.3 5.15
Change-Id: I07321ad91be5adce524be18e4ab82eee7110dc6a
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
5d228beb52