Port [i] to .at(i) to avoid unwanted detaching.

Backtrace:
QList<KTimeZone::Transition>::node_copy(QList<KTimeZone::Transition>::Node*, QList<KTimeZone::Transition>::Node*, QList<KTimeZone::Transition>::Node*)
QList<KTimeZone::Transition>::detach_helper(int) (this=0x1208d928, alloc=126)
QList<KTimeZone::Transition>::detach_helper() (this=0x1208d928)
QList<KTimeZone::Transition>::detach() (this=0x1208d928)
QList<KTimeZone::Transition>::operator[](int) (this=0x1208d928, i=72)
KTimeZoneData::transitionIndex(QDateTime const&, int*, bool*) const
[...]
KDateTime::operator==(KDateTime const&)
(from ical parsing)