QTimeZone - Fix Mac Transistions

Fix nextTransitions() where there are no next transitions and improve
performance of previousTransition() and data().

Change-Id: I5d44525554243d139ba8d5f86e41bdfc0f7bfc25
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
QTimeZone - Fix Windows Transitions

The Windows tz transition routines were not checking for a number of
invalid scenarios, in particular where there are no next transitions
able to be calcualted, leading to infinite loops.

Change-Id: I262b4321a95be1df4228774ada3908f8d3ed6c1a
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>