avformat/matroskadec: Improve error/EOF checks III

avformat/matroskadec: Improve error/EOF checks III

Up until now, when an element was skipped, it was relied upon
ffio_limit to make sure that there is enough data available to skip.
ffio_limit itself relies upon the availability of the file's size. As
this needn't be available, the check has been refined: First one byte
less than intended is skipped, then another byte is read, followed by a
check of the error flags.

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