lavf/matroska*: DiscardPadding is a signed integer

according to the Matriska Specification
 http://matroska.org/technical/specs/index.html


DiscardPadding is a signed integer.

Tested-by: Jan Gerber <j@v2v.cc>
Tested-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>