mad: ensure we do not confuse the base class with our buffer guard
mad: ensure we do not confuse the base class with our buffer guard

mad expects extra bytes at the end of a buffer (see discussion in
http://www.mars.org/mailman/public/mad-dev/2001-May/000262.html),
and since we inject these without the base class' knowledge, we
need to hide the bodies better.

This fixes an assert at EOS when decoding an mp3 manually without
an intervening mpegaudioparse.

collects/gst-plugins-ugly,Lachann/gst-plugins-ugly,Kurento/gst-plugins-ugly,surround-io/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,sebras/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,jar1karp/gst-plugins-ugly,Kurento/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,Lachann/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,jar1karp/gst-plugins-ugly,knuesel/gst-plugins-ugly,knuesel/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,cablelabs/gst-plugins-ugly,Distrotech/gst-plugins-ugly,GStreamer/gst-plugins-ugly,cablelabs/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Lachann/gst-plugins-ugly,cablelabs/gst-plugins-ugly,jar1karp/gst-plugins-ugly,knuesel/gst-plugins-ugly,krieger-od/gst-plugins-ugly,krieger-od/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,surround-io/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,Distrotech/gst-plugins-ugly,GStreamer/gst-plugins-ugly,Kurento/gst-plugins-ugly,krieger-od/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,collects/gst-plugins-ugly,Distrotech/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,shelsonjava/gst-plugins-ugly,krieger-od/gst-plugins-ugly,sebras/gst-plugins-ugly,Distrotech/gst-plugins-ugly,surround-io/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,collects/gst-plugins-ugly,collects/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,sebras/gst-plugins-ugly,Distrotech/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly