events: fix handling of event with no end date

Fix the sigsegv in edit-dialog and event-widgets when there's no
end-date set.
Set a guard in Utils::gcal_dup_icaltime function.
There's still some problems with the events showing the first time but
not anymore.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=736002

src/gcal-utils.c

@@ -128,6 +128,9 @@ gcal_dup_icaltime (const icaltimetype *date)
 {
   icaltimetype *new_date;
 
+  if (date == NULL)
+    return NULL;
+
   new_date= g_new (icaltimetype, 1);
   new_date->year = date->year;
   new_date->month = date->month;