matroska: set "done" only during resync fail.

Fixes playback of test7.mkv validation test file.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Add FATE test for matroska error recovery.

This is the first 2 MB of the official test7.mkv.
That length seems to be enough to detect the bugs
we had in our code so far.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>