mxf2raw: use map to connect track with raw file
mxf2raw: use map to connect track with raw file

The previous code broke when no frames were read from a track,
resulting in the wrong essence being written to subsequent raw
output files.

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