[mp3_afh]: Ignore junk at the end of an mp3 file.

There are mp3 files containing large areas of zeros at the end of the
file. The old code in mp3_afh.c would include these zeros in the last
chunk of the file.

This leads to unnecessary network traffic as including this area in
a chunk means that useless data is going to be sent to the client.
More importantly, it causes the udp sender to bail out because such
large chunks can not be fec-encoded, even with the maximal number
of slices.

This patch uses frame_start+frame_length instead of the file size as
the end of the last chunk which avoids this particular problem.