[evolution-ews] Make sure we have a tzid before we look for the timezone while retrieving a calendar item

[Or Goshen <ogosh src gnome org>]

commit 0c7194bc9aee82d088eebedc17496bd6d0ea678b
Author: Or Goshen <orx goshen intel com>
Date:   Wed Aug 24 12:10:33 2011 +0300

    Make sure we have a tzid before we look for the timezone while retrieving a calendar item

 src/calendar/e-cal-backend-ews.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/calendar/e-cal-backend-ews.c b/src/calendar/e-cal-backend-ews.c
index 2a12b67..dfa43bc 100644
--- a/src/calendar/e-cal-backend-ews.c
+++ b/src/calendar/e-cal-backend-ews.c
@@ -2872,7 +2872,8 @@ add_item_to_cache (ECalBackendEws *cbews, EEwsItem *item)
 			icaltimezone_free (zone, TRUE);
 		}
 
-		if ((zone = (icaltimezone *)e_cal_backend_store_get_timezone(priv->store, e_ews_item_get_tzid (item))) == NULL)
+		zone = NULL;
+		if (e_ews_item_get_tzid (item) && (zone = (icaltimezone *)e_cal_backend_store_get_timezone(priv->store, e_ews_item_get_tzid (item))) == NULL)
 			zone = (icaltimezone *)icaltimezone_get_builtin_timezone(e_ews_item_get_tzid (item));
 
 		if (zone) {