Fix crash when handling timezones without DST.
Fix crash when handling timezones without DST.

The Exchange server takes either *both* Standard and Daylight TimeChangeType
elements, or neither of them. If a zone doesn't have DST, we should just
set the BaseOffset and nothing more.

We definitely *shouldn't* crash, when dereferencing a NULL xdaylight
pointer.

GNOME/evolution-ews,GNOME/evolution-ews