matroskademux: only unlock pad when it was locked
matroskademux: only unlock pad when it was locked

This fixes the mutex being unlocked too much and ending up allowing
other threads when they should not.

https://bugzilla.gnome.org/show_bug.cgi?id=671776

freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good