demux/mkv: fix 17572: error on cluster without timecode
demux/mkv: fix 17572: error on cluster without timecode

As can be read in the MKV specification, a cluster must include a
timecode element in order to be valid.

These changes make sure that we error if such element is missing,
effectivelly preventing us from calling KaxCluster::GlobalTimecode
(which will assert if KaxCluster::InitTimecode has not been called).

fixes #17572

Signed-off-by: Steve Lhomme <4317e33bca9d17016d37cb2d32c67dc65ca52d8e@videolabs.io>
Signed-off-by: Thomas Guillem <5f50a84c1fa3bcff146405017f36aec1a10a9e38@gllm.fr>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc