vss: Rework fec client setup.

vss: Rework fec client setup.

The current fec code assumes that the chunks of the audio file form a
contigous buffer. At least for aac/m4a this is not true, which is
why streaming m4a files over udp never worked well.

This patch should be a big improvement in this regard. We now copy
the chunks to preallocated buffers, which also makes the code easier
to follow because we can get rid of the two extra buffers in struct
fec_client.