icydemux: Fix segfault if metadata-interval is 0
icydemux: Fix segfault if metadata-interval is 0

Prevents an extra unref of GstBuffer when passing a non-icy stream through
icydemux with metadata-interval set to 0.

Reproducible with:
gst-launch-1.0 filesrc location=~/testsong.mp3 ! \
'application/x-icy,metadata-interval=(int)0' ! icydemux ! decodebin ! wavenc ! \
filesink location=~/testsong.wav

https://bugzilla.gnome.org/show_bug.cgi?id=748024

GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,ikonst/gst-plugins-good,pexip/gst-plugins-good,shelsonjava/gst-plugins-good,Kurento/gst-plugins-good,kittee/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,ikonst/gst-plugins-good,kittee/gst-plugins-good,rawoul/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,stfl/gst-plugins-good,GStreamer/gst-plugins-good,sebras/gst-plugins-good,sebras/gst-plugins-good,Kurento/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,vatavuserban/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,shelsonjava/gst-plugins-good,chamois94/gst-plugins-good,chamois94/gst-plugins-good,vatavuserban/gst-plugins-good,shelsonjava/gst-plugins-good,hizukiayaka/gst-plugins-good,ikonst/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,rawoul/gst-plugins-good,stfl/gst-plugins-good,vatavuserban/gst-plugins-good,rawoul/gst-plugins-good,GStreamer/gst-plugins-good,froggatt/gst-plugins-good-m,kittee/gst-plugins-good,pexip/gst-plugins-good,sebras/gst-plugins-good,froggatt/gst-plugins-good-m,froggatt/gst-plugins-good-m,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,rawoul/gst-plugins-good,kittee/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,vatavuserban/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,chamois94/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,shelsonjava/gst-plugins-good