vss: Handle empty chunk groups gracefully.

vss: Handle empty chunk groups gracefully.

The assertion in compute_group_size() triggers if the stream is
positioned right at the end of the file when the next chunk group
starts. This was observed with an mp3 file and the udp sender when
pausing and restarting the stream at the end of the file.

Fix this braino by returning EOF instead of aborting if the next
chunk group happens to be empty.