Backport of "Splitmuxsink: fix bogus fragment split" into 1.18

Backport of !833 (merged) "Splitmuxsink: fix bogus fragment split".

Commits:

• df8b147e splitmuxsink: Convert asserts into element errors.
• 67f70af1 splitmuxsink: Add debug for fragment opened/closed msgs
• d7a9a844 splitmuxsink: Fix for 'reference bytes muxed' check.
• 2d24a45c splitmuxsink: Unit test - check format/opened/closed sequence