demux: mkv: don't use EbmlDummy elements coming out of FindNextID()

FindNextID() is supposed to return an element of the given type when it's found. But in some cases, when the ID and sizes are plausible, an EbmlDummy is returned [1].

We should not use that element as if it was a legit element we're looking for. This is especially crucial when we're opening a file to decide if it's an EBML file or not (EbmlHead).

[1] https://github.com/Matroska-Org/libebml/blob/1c4e2f31b8df7f2c137d8943c73385759aae35b9/src/EbmlElement.cpp#L185