pine-importer: Fix a crash when importing with no existing address books

This is a theoretical crash (not actually reproduced), caused by the
client variable being NULL if no address books exist. Instead of
crashing, gracefully exit with an error.

Found by scan-build.

https://bugzilla.gnome.org/719540
icalendar-importer: Fix a crash when importing with no existing cals

This is a theoretical crash (not actually reproduced) caused by calling
g_object_ref(source) when source is NULL, which can happen if no
calendars or task lists exist.

This fixes the crash, but there may be UI bugs if this condition is
reached.

Found by scan-build.

https://bugzilla.gnome.org/719540