mxf reader: fix read limits in sequence reader for single segment
mxf reader: fix read limits in sequence reader for single segment

The precharge frame count was missing from the duration when the
sequence consisted of a single segment

stuarthicks/bmx,Limecraft/ebu-bmx,ebu/ebu-bmx,kierank/bmxlib-bmx,stuarthicks/bmx,Limecraft/ebu-bmx,xunchangguo/BMX,stuarthicks/bmx,xunchangguo/BMX,ebu/ebu-bmx,kierank/bmxlib-bmx,Limecraft/ebu-bmx,kierank/bmxlib-bmx,ebu/ebu-bmx,xunchangguo/BMX