Backport of "matroska-mux: Fix sparse stream crash" into 1.18
Backport of !788 (merged) "matroska-mux: Fix sparse stream crash".
Commits:
- 0e84f420 matroska-mux: Fix sparse stream crash
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Backport of !788 (merged) "matroska-mux: Fix sparse stream crash".
Commits: