matroska-ids: Fix uninitialized memory in contexts
matroska-ids: Fix uninitialized memory in contexts

https://bugzilla.gnome.org/show_bug.cgi?id=797327

GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good