matroska: read: fix tag list memory leak
matroska: read: fix tag list memory leak

gst_toc_entry_merge_tags makes a new ref of the taglist, so it should
be unref'ed as soon as the tags are merged to the tocentry

https://bugzilla.gnome.org/show_bug.cgi?id=753904

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