avformat/matroskadec: Avoid undefined pointer arithmetic

avformat/matroskadec: Avoid undefined pointer arithmetic

The Matroska demuxer currently always opens a GetByteContext to read the
content of the projection's private data buffer; it does this even if
there is no private data buffer in which case opening the GetByteContext
will lead to a NULL + 0 which is undefined behaviour.
Furthermore, in this case the code relied both on the implicit checks
of the bytestream2 API as well as on the fact that it returns zero
if there is not enough data available.

Both of these issues have been addressed by not using the bytestream API
any more; instead the data is simply read directly by using AV_RB. This
is possible because the offsets are constants.

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