Fix critical error output in the case that an Initialization block does not include a URL.

Issue evinced by
http://yt-dash-mse-test.commondatastorage.googleapis.com/car-20120827-manifest.mpd
produces output like ** (gst-launch-1.0:8060): CRITICAL **: gst_mpdparser_get_initializationURL: assertion `InitializationURL->sourceURL != NULL' failed