demux/mkv: return 0 when the data was not fully read

This is how the IOCallback abstraction works ¹. Either you read the amount requested, or you return 0.

vlc_stream_Read() reads the whole amount of data, unless it reaches the end of file. In that case it returns the amount it could read.

1. https://github.com/Matroska-Org/libebml/blob/ba5707e3268be330516c678c7de14af734f0f3b9/ebml/IOCallback.h#L30 ↩