Detect and skip mj2-style jp2c header.

This this fixes one of the issues with
http://blogs.bu.edu/mhirsch/files/2012/08/Rotate180CCW-checkerboard.mj2_.zip
and at least allows to demux the file properly instead
of failing during probing.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
