reader: fix index for multi-partition files
reader: fix index for multi-partition files

Firstly, must seek back to the first partition containing essence when
completing the essence indexing. Secondly, the end_of_essence only
return true if the position is beyond the known indexed duration.

Limecraft/ebu-libmxf,Limecraft/ebu-libmxf,Limecraft/ebu-libmxf