manager: fail when client unit isn't found

This is a temporary solution to avoid a crash that
some people are experiencing. More investigation is
required.

https://bugzilla.gnome.org/show_bug.cgi?id=781071

src/gcal-manager.c

@@ -1716,6 +1716,8 @@ gcal_manager_create_event (GcalManager *manager,
   component = gcal_event_get_component (event);
   unit = g_hash_table_lookup (manager->clients, source);
 
+  g_return_if_fail (unit != NULL);
+
   new_event_icalcomp = e_cal_component_get_icalcomponent (component);
 
   data = g_new0 (AsyncOpsData, 1);