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

(cherry picked from commit 49d4586f)