The CamelEwsMessageInfo can contain NULL change_key member, probably
caused by other crash.

(cherry picked from commit f1269ab1)

Free/Busy data are supposed to be always in UTC, thus do not use
the 'default_timezone' for it.

Offline GAL, with all data downloaded locally, can be browsable, aka
users can see all the contacts in UI without searching for anything.

This expect only 1..5, inclusive, but the libical function can
return also negative values, which this change takes care of.