This affects the following samples:

samples/ffmpeg-bugs/roundup/issue1775/av_seek_frame_failure.mxf
samples/ffmpeg-bugs/trac/ticket1957/16ch.mxf
samples/ffmpeg-bugs/trac/ticket5016/r0.mxf
samples/ffmpeg-bugs/trac/ticket5016/r1.mxf
samples/ffmpeg-bugs/trac/ticket5316/hq.MXF
samples/ffmpeg-bugs/trac/ticket5316/hqx.MXF

Some AVPacket->pos values are changed because for frame wrapped tracks we point
to the KLV offset and not the data.
Signed-off-by: Marton Balint <cus@passwd.hu>