matroskademux: Drop any block before a valid cluster

This currently results in sending data buffers before a segment event, which isn't valid in GStreamer.

It can be reproduced with a sender like:

gst-launch-1.0 videotestsrc ! matroskamux streamable=1 ! tcpserversink host= port=8888

And a receiver:

gst-launch-1.0 tcpclientsrc port=8888 ! matroskademux ! fakesink

Note that I'm totally uncertain if this is the right way to fix this. I also othough of pushing an early segment event, but it seems to break later streaming.

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