matroskadec: Fix overflow introduced in a569a7b3

matroskadec: Fix overflow introduced in a569a7b3

This commit fixes an overflow introduced in a569a7b3 that affected EBML
elements that the Matroska demuxer doesn't want to parse like CRC-32
elements. The return value of avio_skip (the new position on success or
an AVERROR on failure) has been assigned to an integer which meant that
new positions in the range of 2GB to 4GB-1 etc. were considered errors.

Fixes ticket #8001.

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