avformat/matroskadec: fix DiscardPadding element parsing

If the value is negative then it means padding at the start of the packet
instead of at the end.

Based on a patch by Hendrik Leppkes.

Reviewed-by: James Zern <jzern-at-google.com@ffmpeg.org>
Signed-off-by: James Almer <jamrial@gmail.com>
