matroska: fix memory leak
matroska: fix memory leak

After adding to tag list, key_val is not being free'd
resulting in memory leak

https://bugzilla.gnome.org/show_bug.cgi?id=752992

sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,ikonst/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,ikonst/gst-plugins-good,rawoul/gst-plugins-good,rawoul/gst-plugins-good,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good