(fix) avoid saving non-parsable events/tasks

SoObjects/Appointments/SOGoAppointmentObject.m

@@ -2503,6 +2503,14 @@ inRecurrenceExceptionsForEvent: (iCalEvent *) theEvent
   
   rq = [_ctx request];
   rqCalendar = [iCalCalendar parseSingleFromSource: [rq contentAsString]];
+
+  // We are unable to parse the received calendar, we return right away
+  // with a 400 error code.
+  if (!rqCalendar)
+    {
+      return [NSException exceptionWithHTTPStatus: 400
+                                           reason: @"Unable to parse event."];
+    }
   
   if (![self isNew])
     {