matroskademux: Fix cluster searching if we search multiple times in one chunk

matroskademux: Fix cluster searching if we search multiple times in one chunk

After finding a cluster id in the byte reader, we skip ahead the reader
position by one further byte to be able to continue searching from there
inside the same chunk if the cluster candidate was a false positive.

We have to accomodate for that additional byte when resuming the search,
otherwise all following pulls are off-by-one for every resume and we run
into an assertion.