avformat/matroskadec: Make reading zero-length elements spec-compliant

avformat/matroskadec: Make reading zero-length elements spec-compliant

For a very long time, the payload of integer and float elements had to
have a length > 0. Our parser treated such invalid elements as having a
value zero. But now it has been defined what an EBML element with length
zero means: It is a shorthand for the default value. This has also been
defined for strings (both ASCII and UTF-8). This commit modifies our
parser to support this.

Reviewed-by: Ridley Combs <rcombs@rcombs.me>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>