live555: Avoid instabilities in MPEG2TransportStreamFromESSource

 - A code path exists that will invoke
   "InputESSourceRecord::askForNewData" after its
   destructor got invoked. Add a simple check to
   avoid any instabilities by trying to access
   released resources.

Change-Id: Ibf01b860c6f3a30b5ea5e6f5b7033e52fc335f29