rmdemux: fix tag memory leak
rmdemux: fix tag memory leak

tags is not being freed after being merged to the
pending_tags.

https://bugzilla.gnome.org/show_bug.cgi?id=752404

GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Kurento/gst-plugins-ugly,Kurento/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Kurento/gst-plugins-ugly,GStreamer/gst-plugins-ugly,StreamUtils/gst-plugins-ugly