[3.0] contrib: ebml: fix leak of upper elements

This can happen in damaged files. Undamaged files are unaffected.

Sent for review upstream in https://github.com/Matroska-Org/libebml/pull/321 / https://github.com/Matroska-Org/libebml/pull/322

(cherry picked from commit cace91ae)

Draft on 3.0 until it's merged upstream. A leak is better than breaking playback for some files.

Backport of !7496 (merged)