avformat/matroskadec: Redo EOF handling

avformat/matroskadec: Redo EOF handling

This commit closes the last hole in the system of checks for a
known-length file ending too early: Now an error message is emitted
in case the file ends directly after an EBML element.

Furthermore, this commit adds a check and a corresponding warning
whether there is data beyond the Matroska segment (only reasonable for
known-length segments). If everything looks alright, then parsing is
stopped as soon as EOF is reached (in contrast, the earlier code would
always call matroska_resync at the end).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>