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

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

(cherry picked from commit 49d4586fe82aa105ebc1f519e8c8b7385f89c211)

Gbp-Pq: Name 0107-demux-mkv-don-t-use-EbmlDummy-elements-coming-out-of.patch